I primarily use a nearly-bottom end android phone that's a few years old and just recently switched to an even older, even lower end android phone that is six years old. Neither has that issue.

Obviously, I'm not really claiming that it's not possible people are experiencing this issue, but it can't possibly be widespread.

I feel like most likely people are using android skins that aggressively kill apps in the background.

nope, iphone here, and quite recent. But it's not just me, all the people i communicate with on this app have the same kind of problems. With a group of friends we even had a totally weird ordering of messages, making the conversation quite absurd.

There's something deeply wrong with the way signal delivers messages...