I appreciate your concern, but it’s been 3 months, and it’s still holding a good charge.
I suspect it was a single event where it overheated and gassed. I kept using the phone for weeks thinking my phone case had deformed (I didn’t notice the small bulge, but the case’s power button wasn’t aligning well with the phone), before I realized it was the battery.
When it dies, I’ll get a new phone. If I still have both hands.