1 earbud keeps being low battery, even after charge. Bud or case pb?

I got the Sony XM4 for almost 3 years. For 1 year now, one of the earbuds was constantly lower on battery. It would consume its battery very fast, even after charge. I initially adapted my use (more frequent charging) but now it's unusable.
I came to think it is a connection case/bud problem (in addition to a degraded battery in the bud, due to the constant charge/ uncharge). My guess is that the bud doesn't deactivate while in the case, and the charge pin is not at its best. for the record, I cleaned the connectors on both sides regularly to see if dust could cause that.

I could replace the battery of the earbud first, but I doubt it will solve the problem. What to do with the case? replace the whole connector pad? replace the PCB as well?