Well at least I know not to upgrade my 520+ to the 530, as it sounds like it has exactly the same problem.
Used to be fine, apart from in consistent rain (like my old 800) it’d stop recording altitude gain.
Then it would stop recording altitude gain within an hour of the ride start, or only record about 50% of the total after a certain point.
Worse when it’s wet but still happens on dry rides.
Need to get round to contacting Garmin CS, they have been very diligent in the past and there’s probably some elaborate recalibration technique they’ll make me do – which may or may not help.
Anyway, I tend to mainly just use Strava on my phone now, I’m so cheesed off with it.