I couldn't find any official documentation about this issue.
My iPhone SE (2020) hasn't been displaying the correct battery percentage since I upgraded to iOS 27. The adapter gets warm while charging, as expected, which indicates that the phone is actively drawing power, but the battery percentage doesn't increase.
At first, I assumed there was an issue with my charging cable or adapter, but that doesn't appear to be the case. The issue usually resolves itself after I disconnect the cable from the adapter and reconnect it, or after I restart my iPhone.
There may be a bug related to Optimized Battery Charging. After my phone stopped charging, the battery level showed 38%. However, when I disconnected and reconnected the charging cable, the battery percentage immediately jumped to 80%.
The battery percentage appears to behave normally while I'm using the phone, so I don't suspect there's an underlying battery problem. It seems more likely to be a temporary software or battery-calibration issue.
Major iOS updates can also trigger substantial background processing and indexing immediately after installation, which may temporarily affect battery behavior and charging.
I forgot to leave my phone on for a while after upgrading to iOS 27, so it's possible that it is still performing background processing or indexing. I'll continue monitoring it to see whether the issue resolves itself once that process is complete.