Weird battery behaviour, oneplus 9 pro

[u/Clanceeinfinity]

oneplus 9 pro, crdroid 12.11 (android 16) but the issue also was on, lineage os 23/22, no root. so when i got the 9 pro, it was used and the battery was super off. It lost its charge EXTREMLY fast, like 100-1% in a few minutes. But then 1% would last a few hours. After a bit of putting up with that, i decided to replace the battery with an official ifixit part. What was weird instantly to me was that the battery health didnt start at 100%, but i guess thats normal with replacements. But then after a few months, the battery reports as 55% health AGAIN and starts showing the same 100-1% drain AGAIN. On a new battery... How does this make any sense?