You probably mean the “background usage” exception for apps with Optimized/Unrestricted value. I am using the Android “Battery Saver” / power saver and to my knowledge there is no ability to add exceptions to it (please correct anyone otherwise).
With battery save, you always get "Waiting to exit power save mode”, before Nextcloud sync starts.This means I would need to completely turn off this feature saver only to use Nextcloud.
To give a positive counter-example: Syncthing on Android provides a toggle to manually start sync given power save mode. It’s called “Force start ignore run conditions”.
Also using F-Droid 
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I think Unhappy with the recently lost file upload feature in the Nextcloud app for Android? So are we. Let us explain. sounds like it was your issue. This one might be orthogonal to it.