At the time of uploading, if the phone is in Lock or App background, then the upload stops automatically.
Can you check your configuration at
Settings > Automatic Locking? It could be the expected behavior depending on how you’ve got your app set up.
When the phone is auto locked, the upload is stopped and the automatic lock is turned off in the app.
Maybe the auto locking has locked the vault? If you want, you can increase this number: Settings — Cryptomator 1.5.0 documentation
In the long term, we want to have auto-upload vault separate from the other vaults in relation to auto-locking mechanisms, but this has not yet been finalized and implemented.