Not the main culprit, but a problem none the less:
A lot of indie projects, especially those suddenly exploding in popularity, seem to also suffer from "osmosis" fandoms, where a significant enough portion of the "fans" will only hear about it/get in through the fan media they interact with, while showing little to no interest in the source media before joining.
Then when they actually start getting into the actual source, they are angry/disappointed the source material isn't the same as their "fave" fandom work.
Fandom blind has been a thing for ages, not a problem.
But like it was mentioned, fandom entitlement is a huge problem, and it seems that in the case of fandom-blind people, those feel entitled to get fed exactly why they got interested in THE FANDOM (Not the source), which till then has been nothing but fan works and art. It's that really weird disconnect, where they don't even seem to understand that fandom creations can be different from the source media.
(It becomes even more ridiculous when the fandom does feature very similar works to the actual source, but the fandom-blind people only consumed the AU style stuff, and avoided, or even downright hated the more "canon compliant," fandom works.
And I don't think being fandom blind is the main problem at all! But I wouldn't be surprised if some, SOME, of this entitlement has been further erroding fandom etiquette and growing that entitlement, and has had a part in how people "consume" media, rather than actually enjoying source media or fandom, or both (duh) for the sake of community and enjoyment, rather than just a quick fix and their fast lived appetite.
Just generally in fandom:
Etiquette, and just basic human decency seems to have gone out the window, and instead every little original artist and writer gets treated like a content machine rather than a human being, who should ask "how high" when entitled fans tell them to jump.
These are actually very good points and they bring to mind my own interactions with series that I don't actively get into.
A commonly used phrasing was 'Fandom-Osmosis' to explain how people were able to experience the basic building-blocks of other fandoms and their core IPs by nature of just being online. Major examples being the heyday of My Little Pony: Friendship is Magic, Undertale, and D&D.
An off-shoot of this was using fanworks to get into an IP while not actively engaging with the IP directly. For example, when MTV's Teen Wolf was popular on Tumblr, I was somewhat interested because, hello, werewolves, but I wasn't encouraged enough because I knew that the bulk of the series was actually teeny-bopper drama. So I mostly sought out Teen Wolf fanfic on AO3 instead.
However! It needs to be noted: I never did this with the mindset of 'oh, because I'm engaging with fanwork, I'm getting the same level of depth as people who actually watch the show' or even 'the fanworks are better than the main show anyway, so...'.
Those last two mindsets are what is currently another issue with modern fandom because, as you mentioned, you have people opting for fanworks/fancanons and then getting upset when, upon actually engaging with the core material, they don't see the fancanons on the screen.
And I don't want anyone to read this and think 'oh, fanworks and headcanons/fancanons are the issue'. No, not at all! Those are key pillars as to what keep fandoms alive. However, willful ignorance and entitlement can ruin everything for everyone. Even when it comes to something as 'silly' as being fandom-blind.