Turn off chat heads is the only fix I know of. My brother has been complaining of the same thing on his s20 ultra. I don't like chatheads in general so mine have always been off and haven't had the issue because of that

Unlocking the phone brings the Facebook Messenger bubble forward. Time since last message received doesn't seem to matter. 

From what I can tell it happens if I haven't opened an old message and get a new one from someone else. 

Eg. Person 1 messages at 10am, unlock phone and don't open message. Person 2 messages at 12pm, unlock phone at 1pm, person 1's message is now open and in focus. 

At the point where I'm ready to uninstall messenger. Anyone else experiencing this? Is there a known fix? Is it down to Facebook to fix?

Unlocking phone opens messenger