Why does my Alta HR lose charge even with a new battery and ios 16?

Is anyone else having issues with their Alta HR recently?

In the last few months, my Fitbit Alta HR has not been behaving consistently. It dies within a few hours so I can no longer track my sleep, it shuts off even at 75% battery, and sometimes it doesn't charge all the way to 100% (only to 80%). I was using an iphone x with ios 14.

I replaced the battery with a BRAND new battery, only to find that the same problems are happening! And I updated the fitbit app and my ios to ios16 and am still noticing a short battery life.

I worry there is a software / firmware issue that Fitbit won't fix since it is phasing out Alta HRs and only allowing ios15 onwards. Does anyone have suggestions on how to fix this? I've loved my Alta HR and the sleep tracking has been critical for my health so need to find a replacement sleep tracker asap.