I had exact the same issue!
Resetting Phone was not helpful, it was mostly unusable when the battery drops below 30%!
In the end, I replaced the phone within warranty but still happened, so I changed the battery and it´s now working fine!
As those batteries are "intelligent" they are not very clever.
try to get an original battery for example Amazon.
check how long it takes to load. if it is too long, the battery is old and way laying around too long at Amazon.
Return/ dispute if necessary.
Best Regards
Daniel