Hello & Welcome to Just Answer,
Sorry I didn't see your question earlier.
That have been a few posts about this issue across the internet, with some even returning their devices to Garmin for replacement a number or time, only to see the issue return with each replacement - over time.
What I believe is the "fix" for this issue is "cleaning of device" (especially around the barometer sensor holes) with warm soapy water and a clean or unused toothbrush. And of course you should let the device dry off fairly well before TESTING the elevation functions again.
The idea is that some "dirt" (sweat, chlorine, river water, etc.) gets into the holes and blocks them up.
I will try to get an image of the hole locations for you, but they are on the back edge of the device. From what I understand, it may take a couple of cleanings to get really good (stable) results.
Please give that a try and let me know your results.
Thanks, ***** *****