Apple Watch Model MTF02LL/A face detached
My warranty has expired. Series 3. How do I know whether the battery has swollen?
Apple Watch Series 3, watchOS 8
Has the battery ever been replaced by Apple?
Depending on the age and use of the watch, an expanding battery can push against the screen of the watch, causing it to come off.
Apple can replace the whole watch, if you are willing to pay the service fee:
Apple Watch Service and Repair - Apple Support
Buying a newer watch may be a better way.