I have had my watch in Airplane and I have torn through 45% battery.
Take a look at this screen shot.
Airplane mode on but I am connected to the phone and locations.
As referenced many times in recent weeks, Airplane mode remembers whether or not you'd enabled Bluetooth in Airplane mode. See https://support.apple.com/en-us/HT204234
You don't mention how long off charger or whether you've done workouts since then.
BTW - since GPS is a receiver, an app which turns on GPS in Airplane mode would presumably work fine. Might take longer to get an initial fix. Haven't really experimented with it but nothing should preclude it from working.