Since the update to UI my open apps shut down automatically. I leave some apps open and like it that way. Now after a time, I go to seek open apps and they are not there. This also leads to second problem that open apps are no longer in one cascading window but side by side windows that you have to scroll through. What used to be a one step process is now a multi step action. So many annoying changes.
Have you tried going into your settings to stop the apps being put to sleep i.e. stop doze.
It's a feature of the phone to clear ram if needed which is accomplished by closing down suspended apps.
Current Phone ~
Samsung Note 20 Ultra 5G SM-N9860_256Gb SnapDragon 865+
Samsung One Ui 3.1/ Android 11.
Same problem to me as well. The applications are getting closed automatically.
Some times, it is annoying. While typing some thing in samsung not as well, it is just getting closed.