Weird I have never experienced this (or it was so long ago I don't remember). We've been using Signal absolutely reliably for our family Chat for many years now, and also with other friends.

I don't think Google Play Services are required, but

your power optimisation settings might prevent signal from running in the background

could have sth to do with it. And that's accessible through Settings, an App Permissions thing IIRC (although I never needed to do that manually either).

PS: I sent you a dm on a different matter.