Bluetooth schedule - In Extended Mode, the underlying Wear OS system software sometimes enables Bluetooth use even though it is scheduled to be off which imacts the battery life. This may also result in having the watch switched into Custom Mode if the Smart Battery Modes app is launched when Bluetooth is scheduled to be off but has been overwritten by the system software.
Settings change after charging - If your watch switches to Extended Mode automatically, the system does not retain your previously selected settings. You should manually switch back into Daily Mode if you have not changed any settings.