The thing is that Yuri is only especially autistic when she has to interact with people outside of her friend group. When she's just hanging around with Tomoko, Mako and Yoshida she's perfectly capable of acting like a mostly functional human being (at least until Tomoko starts making friends and Yuri starts acting like a jealous sperg). It's just that prior to the party we never really saw her interacting with anybody aside from the field trip group. In fact one of the few interactions she DID have with anybody else basically has Ucchi screaming at her internally for acting like a standoffish loner.
That isn't to say that her character didn't change at all, it's pretty clear that the authors' current image of Yuri seems quite different from how she started. Her body language was definitely a lot more "casual" than her current kind of awkwardly tense demeanour, and she definitely seemed to smile a lot more easily as well. And her being so quick to warm up to Tomoko and Yoshida definitely kind of conflicts with her current attitude (You can maybe justify it as her being quicker to open up because of her earlier fight with Mako but that definitely just seems like kind of an in-retrospect justification for something that more likely just comes down to them not having as solid an idea of her personality). Interestingly her design also shifted to become a bit less "plain" and a bit cuter around chapter 100 or so, I wouldn't be surprised if that signifies the point at which they gained a much more solid image of Yuri's character and what they wanted to do with her going forward, especially since her design has remained mostly consistent since then.
(Also have a Yuri, friends).