Apps closing by themselves from the recents

Hello! So I have this problem I am not sure if it is a problem or something normal that's why I want to ask. Sometimes when I open more than four apps they start randomly closing out of the recent apps page you know the one when you tap the square on screen button (usually). I mean I don't close them I leave them there and they stay for awhile and after some time using my device when I go back to the recent apps page one or several of the apps are gone and I haven't closed them. I mean is...

Apps closing by themselves from the recents

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