It doesn't really appear that there's a widespread bug related to that. There's really also no way to completely restrict apps from running in the background at times after they have been used at some point, short of actually closing them (or never using them to begin with).
No, you're right, but on 13.5.1 nowadays, certain apps really keep running in the background as long as you don't give them a forced shut down. I use 3 of those apps since iOS 11 and they never had that specific behavior. It's introduced since 13.5.
If it's not present anymore in the bèta of 13.6, I could use that bèta...
A pity: I don't own a spare iPhone to check that out...