Galaxy S6 Edge :: App Windows Keep Closing Randomly?

I've been noticing this happening for awhile now.. for some reason my app windows will close randomly- no warnings, no errors, i tap the Recents key and apps that i had open are now gone.

There doesn't seem to be any pattern or consistency to this, i'll just eventually notice after doing things in other apps, that when i tap the Recents key, one (or more) have closed- in some cases i've had multiple apps open and they've all closed- ie, tap the Recents key and it's blank.

For example, just recently i had the Facebook app open (along with a few others), and when i went back to open it from the Recents key, it was gone. i've also noticed the same thing happen with the Instagram app, but i believe there are others..

So I'm assuming this isn't supposed to happen? my only thought was maybe apps are crashing when too much stuff is open at once, thought it's a pretty high end phone, and i typically only have 3-4 apps open at the same time, at most.. or maybe apps are closed automatically if they're open for too long with no activity? i can say that in some of these cases the apps had been open for hours or in some cases days by the time i noticed they were closed..