Come on, man!
This is an ongoing issue that multiple users have reported and the other threads were closed with no answers.
Facebook messenger popping up and even opening on launch after having been deselected multiple times is extremely intrusive. Have you forgotten why so many choose Opera?
Well, hopefully you get this fixed before you lose more users. It's too late for me, I'm off to try Vivaldi and I probably won't ever be back. Shame!