I'm going to use Good Lock on my Flip 3 to try and get similar results to Cover OS. I'm already using it for a custom wallpaper... I trust it more since it's supported by Samsung. I'm not a big fan of third party apps on the whole... seems like they're usually responsible for a majority of phone issues
If it's still happening then unfortunately there's still a problem. If it doesn't happen at all, ever, in safe mode then it's a third party app problem most likely. But since it's still happening to some degree that's kind of an early warning sign
It might be a good idea to back up everything on the phone just in case

I've heard of Flips having that as an early sign before screen failures
If it's an app problem you could start removing apps you've added from the most recent back, until you find the app causing the problem. Or you could factory reset to rule out any software problems period.
I wouldn't want that problem to be going on with my phone, because you never know when it's possibly going to quit and leave you high and dry