My iphone 11 goes off even when battery storage is 80%

My iphone 11 battery health is currently at 70, and it goes off after the battery runs down to 80 %. when I put it back the battery storage comes to 10%. Do I need to replace the battery?

I watch movies on the phone and sometimes plug it in to charge while using it at the same time. I suspect this may be the problem.

Any suggestions to resolve this issue?