I have it too a strange problem with my battery.
Often the morning when I switch on the phone the battery from 50-60% of night drop down to 10-20% at the morning.
It’s a strange issue that I’ve learned to prevent by switch off my phone during the day to see the “really” level of charge of battery.
I tried twice to force the software reset (from 0% to 100% as explained here) but it didn’t resolve.
I tried also spin test and I can see that can rotate but it doesn’t look so much swollen and it fit quite well in the battery compartment…