What is wrong with my battery?

My battery randomly drains from time to time (like 1% every 30 seconds) and works perfectly sometimes. A 10 min youtube vid sometimes takes 2% battery while other times takes 10%.

-phone works and functions perfectly

-battery has been calibrated

-charging times are normal ~ 2 hours for full charge

-battery has been recently replaced, perfect wear level on batterylife

-not any apps or anything (checked THOROUGHLY)

-low power mode doesnt change anything

-drains 10% to 30% overnight (~8 hours) no use, no notifications popping up

-turned phone off, waited a few minutes, then plugged charger. It turned on. (if this means anything)

Is this because of a bad u2 ic chip?

Or a bad replacement battery?

Or anything else?