Watch keeps setting Max HR to 185

My experience is that retrospectively changing HRMax in an activity isn't possible, and even if it was, your other data wouldn't be re-calculated.

Do you have any other Garmin devices that may be trying to sync a conflicting HRMax?

BTW, I'd suggest never using the 220-age model, even as a starting point as the margins of error on it are massive. This from a 54 year-old with an HRMax of 200 who wasted many years of his younger life training in entirely the wrong zones due to using this formula. You're better off just giving it your all at the very end of a hard run and if your HR plateaued, take that as your value; if it was still climbing then maybe add another 4-5bpm. Not perfect science, but better than 220-age.

I only have the one device I know what you mean I was happy with where it was manually set for me but it keeps changing it now after a run to 185 for some reason... Luckily I pretty much know my zones anyway but this has only started happening recently no idea why nothing has changed except the updates.

One thing you might want to try is to set the auto update HR to 'on', maybe do an activity, then set it back to 'off' again (and reset your HRMax to what you want it to be). I remember once an update caused mine to think it was set 'on' until I toggled it like that

Thanks I'll try that 

Cheers Ben

maybe double check if you're setting the max heart rate for the actual activity and not the general one.

user profile > heart rate & power zones > heart rate > sport heart rate