krysor while it is possible that there is some bug you're experiencing, I find it more likely that your battery still hasn't fully re-calibrated itself in a long time. My suggestion would be to plug it in at night and when you wake up check if it has reached 100%+Shield.
I have a Pixel 8a as well and just this week had it charge to 100% +Shield and successfully re-calibrate. It's plugged in right now and is showing 80%+Shield as it should.