So glad they are working with you to resolve the problem
If it wasn't for you explaining the software update I wouldn't have known that my oasis 2 is also on the latest 5.9.7 update. I know it must have been done automatically because I didn't update it myself. So I'll be waiting to see if the update was the problem. I know not everyone is seeing it on their device but it seems that just recently my battery is draining more than usual. Not as severe as yours but enough that I noticed it too.
Good Luck !