I'll go first:
It was recently explained to me that there is an unspoken social rule where leaving a social gathering before the first two hours are up is considered rude unless you have a really solid reason for leaving.
I have somehow gone through most of my odd autistic life without knowing this and now I'm wondering what other social rules everyone else seems to know by default that completely flew over my head!
ETA: Thanks everyone! Based on the responses here, I think this "rule" may not be an actual thing and perhaps I have been duped. I suppose, the better way to go ahead with this group of people would be to communicate that while it is lovely to be included in their plans, it is beyond my capabilities as a disabled person with ASD and chronic fatigue to adhere to this requirement of theirs (because I am starting to feel that this is specifically this group's requirement and not an actual, universal thing).
ETA 2: Their rationale for this requirement was that if you go to an event focused on a specific agenda (for instance you went to a birthday and you said your greetings, had the cake, wished them well, thanked them for hosting the gathering and left or say it was for having a few drinks so you interacted with people while having a drink or two and left) and you leave as soon as that focal task is complete, all you did was meet the basic agenda of the evening and you didn't show them that you did care enough to carve extra time out of your busy life for them. While I disagree with this and I feel that my effort to show up should be enough, because I have been silently ghosted from similar groups in the past, I thought that perhaps this was why I kept being ghosted because I didn't bother to stay longer than I was needed to and so maybe I should try to honour this. Who knows maybe that is actually a reason or maybe it isn't but I think being very honest about my limitations would be a better and sustainable way to go moving forward.