>>280182378
>>silently leaves meetup without saying anything
some people are just busy. Saying "bye!" can take forever sometimes, and sometimes people have to bail. 
That's less of an issue than you might think.
>talks to you normally, the next week they act like you're a stranger
ADHD, or too many friends, not a lot of time. Don't take that too personally either. 

Everything else is super valid though, but also remember awkward social people will do some of these things without intending to do that. 

don't attribute to malice immediately.