Wrong battery level on OnePlus8T

Hi,

One morning, my phone does not wake up. I returned it to the seller, he said everything is OK but I asked him to replace the battery.

I received the phone back and everything seemed fine. After a few days, I take my phone after it has charged all the night. 30 minutes later, the battery indicator says 20%…

I charged, 30 minutes later the battery indicator says 100%.

One hour later, it says 1%… I thought the new battery is HS. But, many hours later, the phone was always running, always 1%.

Everything works fine except the battery gauge.

Software issue or battery issue?

Thank you for your help