I think I share (again) this similar headcanon to @travalerray and @sun-lava and maybe a few others on the server...
That Suibian is actually an old blade that had already been well-cultivated by a prior master.
And of course I like to think Wei Wuxian just never knew this fact, because Jiang Fengmian couldn't let it be known he gave his ward a special sword or else his wife would see it as more favoritism. This also explains the overly plain hilt and scabbard, to oversell it as a new blade.
There's a couple of reasons why I think this headcanon works, and why canon doesn't explicitly disprove it:
1) How Suibian can seal itself when no other swords we know of seem to be able to do the same (Suihua and Shanghua both allow other users and Shanghua was a well known enough blade to be entered into a famous sword catalog). That it's considered such a rare event for it to happen.
Also that Suibian seems to be able to recognize a soul, which is different from a spiritual system, which is what the golden core would be a part of. Suibian recognizes Wei Wuxian within Mo Xuanyu's body—that has an entirely new spiritual system capable of solidifying a new golden core if Wei Wuxian worked for it.
I can’t believe it really sealed itself off. I just had to be the one to experience this kind of one-in-a-million event.
This sword was Suibian. This was Wei Wuxian’s sword. It had been taken by the Jin Clan of Lanling after the Siege of the Burial Mounds as a trophy for their collection. No one had ever been able to unsheathe it after that, since it had long since sealed itself of its own accord.
“I also heard that while no one could draw the sword, you [Jiang Cheng] were able to do so. How very curious, indeed. The sword was already sealed when it entered my collection thirteen years ago. Absolutely no one could draw it aside from the Yiling Patriarch himself…”
This fits with Jiang Fengmian's wish to take good care of Wei Ying as well as add to the golden heroic air of xianxia hero teen Wei Ying that a special sword became his.
Sentience that can recognize souls/masters across space and time is usually a very high level thing for spiritual tools for the genre—like Zidian recognizing a hierarchy of masters and obeying commands while at great distances from the master that commanded it. Yet if Suibian were new, it'd only been cultivated for 2-4 years at max and it'd be even more farfetched to believe it capable of Zidian's level of sentience. Like, Wei Wuxian is genuinely surprised it sealed itself...
Corollary Headcanon: This also means that piece of Wei Wuxian's original soul is attached to Jiang Cheng's gifted golden core.
2) The name of the sword is written on the hilt instead of the blade. Fuxue's was as well, so this seems to be common practice in this universe. But! The hilt is a part that can be replaced and equipped to a reused blade.
It follows then that Jiang Fengmian would let Wei Wuxian 'name' the sword if he's playing it off as a newly forged one. You notice some of the wording is funny here like Wei Wuxian says that "when Jiang-shushu was bestowing the sword upon me" only then does Wei Wuxian say 'whatever' and then the sword "came out of the kiln, that would be the word on [the hilt]". Obviously, this can't be taken too literally because hilts are attached after the blade is forged. Also use of the word kiln instead of forge implies casting metal which is a whole other thing. To me it sounds like a process Wei Wuxian wasn't involved in, and didn't see it all be put together.
Within the engravings upon the hilt were the carvings of two ancient characters, and what they spelled was the word “Suibian.”
Wei Wuxian considerately explained, “You don’t have to say anything. I know. You must be wondering why I gave it that name? Everyone asks if there’s a special meaning to it. To be honest, though, there isn’t. It’s just that when Jiang-shushu was bestowing the sword upon me, he asked me what I wanted to call it. I thought of over twenty names at the time and wasn’t happy with any of them, so I thought, why don’t I let Jiang-shushu give it a name instead? So I responded with ‘whatever’! Who would’ve figured that when the sword was forged and came out of the kiln, that would be the word on it? Jiang-shushu said, ‘Since that’s the case, let the sword be called Suibian.’ It’s not actually such a bad name, don’t you think?”
Corollary Headcanon: The blade was Cangse-sanren's, hence it's snowy color (her name means 'hidden color' or 'colorless', you can play with the imagery considering it's a white blade) and vibes with how/why the sword was so utterly loyal. Jiang Fengmian found it when trying to back track to find Wei Ying. Again, the reason he never mentions this is the same reason in canon that he never mentions Cangse to Wei Ying—he just doesn't want to talk about it.
