That has also happened to me. After a full charge, the battery data for the previous days get erased. Happening since 15.0. It happens sometimes, not every time.I had a weird one, I charged my phone last night and lost all my battery data. It shows two days and the last 24 hours, not 10 days. I called Apple who did a diagnostic and all is well with the phone, so maybe some kind of bug is what the Apple guy said as if the battery was having issues it would show up with the test they do. iOS 15 is a bit hit and miss on 15.0.2, lets hope 15.1 sorts many of these issues out.