Altitude reset every night to -800,000 metres

Not a solution to the core problem. But perhaps try saving your home as a GPS location. The watch should then snap to that elevation when you start a run/walk/whatever from that location, and avoid you having to manually calibrate the altitude.

In my opinion it is hardware related, had this problem with my 5x Plus, had to be replaced by Garmin after an attempt to be fixed under warranty.

Contact Garmin Support

Two things to address here.

1 - The watch has a setting for automatic overnight altitude calibration. You can turn it on or off. It sounds like it's turned on, in your case, which is why it's resetting every night. If you turn that off, it will stop resetting the altitude every night. If you're not able to fix this issue after reading the next part of this post, try turning overnight calibration off so that at least you won't have to manually re-calibrate every morning.

2 - Then there's the core issue of why is it auto-calibrating to -800,000 meters, instead of the correct altitude? During overnight calibration, your watch gets the GPS coordinates from your phone (not its own internal GPS). Then the watch retrieves the altitude data for the phone's coordinates from the watch's DEM file, and sets that as the current altitude. So the problem is perhaps that either your phone is giving the watch bad (or no) location data, or perhaps the DEM file in your watch is corrupt. I'm not sure how to fix a broken DEM file, but someone else here might, or Garmin Support could help with that. If your phone is giving the watch bad location data, maybe this can be resolved on the phone side. Is location services turned on all night? Do you have the permission enabled for Garmin Connect to be allowed to use location services "all the time", or only when the app is open? Newer Android version (and possibly Apple as well) have 2 settings for app's to use location services: "only while app is open" or "all the time". If your phone is not letting Connect use Location Services while you're asleep, then that might be the problem.

C.sco said:

During overnight calibration, your watch gets the GPS coordinates from your phone (not its own internal GPS). Then the watch retrieves the altitude data for the phone's coordinates from the watch's DEM file, and sets that as the current altitude. So the problem is that your phone is giving the watch bad (or no) location data, or perhaps the DEM file in your watch is corrupt.

I agree. I noticed that when I set the elevation manually in my home, thereafter the watch usually set that elevation (for the 3rd floor) rather than the DEM elevation of ground level, which is exactly what I want. So it is possible the OP has a duff value stored in his watch that is overriding the DEM. Puzzling that the manual resets haven't overwritten that, though, if so.

Thanks everyone for the replies.  I will try turning off the auto calibration and see how it behaves.  I do have location "always-on" for the Garmin connect app, so it wold seem that is not the issue here.

Con

I have the same problem. Every night (exactly 3 o'clock) my altitude goes up 90 meters. trying to turn off the bluetooth phone the problem disappears. Obviously this is the result of a wrong auto calibration. Considering that probably even the GPS of the phone will have trouble receiving the signal at home at night I wondered what information of the phone is being used by the watch and how it can be adjusted.

Trying to do a manual calibration using DEM everything is ok so I don't think the clock has corrupted DEM data. So where is the problem?

Does anyone have any other information that can help to understand this topic?