I disconnected Apple Health from Cronometer after having to do multiple watch resets and the watch-iPhone connection has been perfect since then. Battery life has been consistently better as well. Probably 3 months or so since I did that.
My experience has been the same.
Cronometer app Apple Health connection enabled eventually causes significant Apple Watch battery drain and delays in health data syncing from Watch to Health app on iPhone.
When Cronometer app is NOT connected to Apple Health, those issues do not appear.
Other apps that I have connected to Apple Health do not result in the Apple Watch issues.
Ok i can confirm now after a couple of weeks without cronometer <-> healthkit link active, that the watch battery drain is back to normal and the watch <-> healthkit sync works without issues again. So the trick was to not use that link at all...
I have noticed that the battery drain seems to creep up over time. When it gets really bad I will unpair and re-pair my watch, or in one case I reset my phone completely from scratch. Once I do that my Apple Watch battery is right as rain for a couple days. This may be why it seems to some people like the recent updates to Cronometer fixed the problem when to others it didn't.
Slowly over the coming days, the battery life starts to suffer until I get to where around midday I'm at 20% and need to charge (I charge fully every night). Not sure if this helps.
This is indeed a very frustrating issue, but I've talked this through with developers from another app I use and I know that Apple's diagnostic tools - particularly on WatchOS - are really limited and make it challenging to debug.
I'm a big fan of Cronometer so I am reverting to turning off Apple Health syncing and I'll manually add calories burned until this is fixed. I've heard that WatchOS 7 (should be coming in a few weeks) should significantly improve debugging, which I'm hoping helps developers like Cronometer better trouble-shoot these problems.
I too have been struggling with this for months and had to turn off syncing with Apple Health. Has anyone tried turning it back on since the release of iOS 14?
Sometime during the iOS 14 public beta my watch started getting better battery life. Still doesn't make it thru the day but it is over 5 years old.