#(for herself)

Because it is merely a game mechanic and Larian did not want your only potential paladin to be an oathbreaker upon recruitment.

Her original oath to Lolth never broke because she never willingly joined the Absolute and her oath to the Absolute was made under false pretenses.

When she joins your group, she makes a new oath in your name instead.

Personally, I think that Minthara's oath is actually a mixture of 2 and 3. Yes, it is possible for a paladin to have multiple oaths at the same time. It just isn't a recommended practice in tabletop DnD because they can sometimes contradict each other, some oaths will straight up break other oaths, or the players just are unable to manage them properly. In this case, I do believe that Minthara has two oaths, one to Lolth and one to you.

I have not been able to find any reference to how paladin oaths work when the paladin is being coerced or mind controlled. But from what I have read in DnD lore, when a paladin makes an oath they have to do so willingly, deliberately, and consciously. None of those things occurred for Minthara when she took up her oath to the Absolute which is why it did not break her oath to Lolth (because it otherwise would have as she would have been sworn to destroy the Absolute for Lolth). And, technically speaking, the god that the oath is sworn to cannot go in and break the oath. Only the paladin can break their own oath. I only think gods can revoke divinity from their clerics. So, even if Lolth was truly pissed at Minthara, Lolth herself cannot just break Minthara's oath. That is something only Minthara can do. Minthara's original oath to Lolth still being intact is not necessarily indicative that she still has Lolth's favor, it just means she hasn't yet broken the terms of that oath. Even a crisis of faith (which Minthara is going through) won't break the paladin's oath either.

Another thing that is important to note is that as of 5e, a paladin actually does not need to make their oaths in the name of a god or before a god as their divinity is powered solely by the oath itself. Paladins are able to just summon divine smites and other holy based powers by sheer force of will and conviction (which is pretty damn sexy if you ask me). I mean, oathbreaker paladins still have access to divinity as well. Of course, paladins can still make an oath in the name of a god and most typically do, it's just that gods are not the source of power behind that oath. An oath is nothing but a vow, a promise that the paladin makes *to* a god, not the other way around. It does not even need that gods approval. So Lolth abandoning Minthara will also not break Minthara's oath either because Lolth is not the one who made an oath to Minthara.

So, it is very much possible that when Minthara joins your group, she does make a new oath in your name as oaths don't need gods. But, Minthara herself does not actually know how to live her life without having some kind of god figure so she just kinda puts you in that spot for the time being. I mean, she literally prayed to multiple gods and you were the one that answered. Which is also one of the many reasons why she does to along with what you want to do (even if she disagrees) because the oath she made to you would keep herself in check and keep her from acting against you. Minthara already is a very loyal person, but if you have any doubts on Minthara's loyalty, just keep in mind that she is quite literally oath bound to do whatever you tell her to do and will not betray you.

Minthara also never goes out of her way to become an oathbreaker, even after you free her from the Absolute. To be precise, you have to command her to break her oath. And of course, she will do so willingly knowing the consequences. Despite her oath being broken, she will still abide by the tenants of that oath. But she does enjoy the freedom that comes with being an oathbreaker and now nothing will hold her back from achieving her goals. However, she also says she does not know if her oath will still stand when all the nonsense with the Absolute is resolved and she even implies that if you do take the Absolute, she will no longer have an Oath of Vengeance, but an Oath of Conquest (which I think fits her better than vengeance if I'm being honest).

Minthara also makes quite a few suggestions to you that would actually break you oath if you are a paladin. Such as making an oath to Gortash, or making certain choices in regards to the vampire spawn. But, Minthara never does any of these things herself and I think that is purposeful. She knows these actions are oathbreaking type of actions. She attempts to orchestrate a fight between Dame Aylin and the wizard knowing fully well that it will break Aylin's oath. And she really wants you to go snitch on the wizard (although I just think it's because of her disdain for wizards rather than her being a little scoundrel to mess with Aylin's oath).

She is very very careful to keep herself in the position that she is because she doesn't want to be an oathbreaker initially. She is also very careful to tell you to do all these things. The language that she uses in regards to her oaths are also very important. Her original oath to Lolth would have her sworn to destroy the Absolute. But, we also know that Minthara very much has other ideas for the Absolute that do not involve destroying it (but oaths only break from oath breaking actions, not oath breaking thoughts). And her oath to you, she very specifically says that she is sworn to destroy all those who serve the Absolute, which is not exactly the same kind of oath she has with Lolth.

But, there is only one instance in this game in which Minthara will willingly become an oathbreaker and you do not have to command her to be one and that is if and only if you claim the brain. Her choosing not to destroy the brain when she had the opportunity to breaks her oath to Lolth. Now, the oath that she makes to you is where it becomes very very tricky. The oath she made to you, she specifically uses the language to "destroy all those who serve the Absolute". Her oath gets broken to you because she has become someone who serves the Absolute. But, there's more than that. If you become the Absolute, you also enthrall all your other companions which would make them servants of the Absolute and she also does not destroy them. Ironically, her choosing to serve you as the Absolute, breaks the oath she made to you because she has gone back on what she said she was going to do in your name. But most importantly, she chose not to get vengeance for herself.

Her oath to you was never to help you become the Absolute. Her oath was specifically to assist you with destroying the Absolute and all those who serve it. Her choosing not to destroy the Absolute and everyone who serves it, breaks both of her oaths. So, the Minthara that greets you at the gallows is now an oathbreaker. But, as I mentioned earlier, Minthara has already expressed the notion of taking up a completely different oath if you were to claim the Absolute. She helped you become the Absolute, knowing it was going to break her oath, and she had the intention of immediately replacing it with an Oath of Conquest, making her the first official paladin of the Absolute.

The fact that she wants you to become Absolute at all and that she prefers being an oathbreaker is why I read her as a canon oathbreaker. But she will never break her oath out of betrayal, but because of loyalty and devotion. She breaks her oath trying to make you as powerful as you can be. I have also discussed previously that her devotion is her fatal flaw and her choosing to remain devoted to you could potentially get her back in the same position you found her (that is if you betray her and choose to use the Absolute to reenthrall her). Her becoming solely devoted to you officially removes any and all ties she has to Lolth. But it causes Minthara to remain stagnant in which she doesn't learn anything, she doesn't change at all, and she doesn't grow.