I've noticed the same thing. A lot of comments on federated posts just do not show when viewed from another instance, but do show up when viewed from an account that is part of that instance the post was made from. It's really annoying and I'm pretty sure it's a common lemmy bug but most people just don't realize it because most people never try viewing the same post with accounts on different instances because there shouldn't be any need to.
It can be pretty drastic too. Like only 4 comments seen vs 40 comments actually being there. It makes me wonder what comments I'm not seeing on other instances and what comments of mine people on other instances aren't seeing.
Yeah, I might do that, but I think they're already aware. I looked at some lemmy help communities back when I first noticed the issue during the reddit blackout, and there were a number of other people already talking about the problem. It was mostly people making their own instances because they were testing their newly-made instances with more established instances and realizing tons of comments were missing depending on which accounts they were logged into.