iPhone 12 Pro Max won't charge past 80%

My iPhone 12promax has been having battery issue lately..and it’s a new phone barely 5months. When charging, once it gets to 80% it becomes difficult to go above.. then just this morning it went off at 56% on its own.. just to turn it back on and it was at 10% and went off again immediately. I used a power bank to try charging it.. it came on at 40%. Am just tired.

[Re-Titled by Moderator]