Bought my first apple watch in March 2024 - series 9 (GPS + Cellular). Company pays for service on both so it's paired to my work phone (company supervised/managed iPhone 14). All was working great for about 5 months. Then boom, suddenly started draining excessively fast - like 100% to 0% in 4-6 hours. Tried resetting sync data with no luck so erased contents/settings and restored from back-up. That seemed to help a little but then went back to the same issue. I kept having to repeat this cycle of erasing and setting up watch (restoring from backup) 1-2 times weekly. As time went on, the issue has gotten progressivley worse - now only getting 2-3 hours from full charge. Was finally able to make it to Apple store in December (I don't live close to an Apple store) to have genius bar look at it. They could see the issue with rapid battery drain but couldn't determine the cause--battery health was 96% so they wouldn't replace it. Genius bar advised to send it off to Apple support to try to diagnose if issue was software or hardware related. Got the watch back in the mail 2 weeks later--nothing was found wrong with the watch. Paired it again to my iPhone 14 and same issue returned as before. I then reset my iPhone and restored from back-up (the genius bar tech suggested this as a last resort if Apple support did not find any issues. Still didn't resolve issue.
Returned to Apple store yesterday, again they could see the issue with rapid drain - said it could be because I was restoring watch from backup which may contain a corrupt software issue--genius bar reset the watch, and set up as a new watch, again paired to my iPhone 14. Didn't install any extra apps when pairing (only the default ones are on there). Genius bar told me to keep any eye on it and if that didn't fix it, there may be a software issue with my iPhone that's causing the issue. Got home an hour later and within 2 hours, my watch was dead. So I decided to experiment - reset watch, paired it with my wife's iPhone 13 Pro Max - guess what, no more issues - has lasted all day today and still at 29%. So now the question remains, what's causing the issue with the battery drain when it's paired to MY iPhone? I feel like the only way to fix this now is to reset my phone again but to NOT restore from backup, which means I loose everything--I can't do that. I'm at a loss - any ideas?