My Icon Of Minimized [Pop-Up] Apps Sporadically Disappears

[frozendonaltobello]

... and I have to individually re-open each window then minimize each one to regain whichever 6 or 7 screens I lost due to the disappearing Icon.

Does anybody know what I'm talking about about and can anybody tell me how to solve this aggravating situation?

I'm using a 6s Lite 

Thanks, community