Despite I hate to watch a (quick produced) video for that little information from the DJI Osmo Pocket FAQ, the charging is not to be blamed on DJI but on Android:
Q: How come the Osmo Pocket keeps charging my phone when connected?
A: According to AOA protocol of Android, Osmo Pocket must provide 5V 500mA power to the Android device when connected. This will result in reduced battery runtime, however, the Osmo Pocket allows you to charge it when it’s turned on. Adding an optional Wireless Module can also prevent the reverse charging from happening while not compromising advanced control and features.