@grillo54 had that happen - sometimes it just doesn’t fix the start altitude. You will probably find that actual ascent and descent is correct (which sounds like it was based on your original post).
You would need to speak to suunto as to why it does that, as I don’t know. Tried numerous combinations, couldn’t get anything to actively trigger the issue and or to fix the issue when it occurred - sometimes it corrected itself sometimes it didn’t. Checking alt before starting, I could sometimes catch and fix it by stopping activity, discarding it, starting up activity again and waiting for fix and a bit longer - sometimes it worked, sometimes it didn’t. As I said starting the activity while still connected to a phone did seem to reduce the occurance when this was happening.
Personally I think it may be due to how its pulling the GPS data, and then aligning it to DEM maps? But don’t know enough about the inner workings.
Altitude issues aren’t unusual - loads of posts about this on all different watches etc. For example I found my F6 was doubling ascent & descent, so changed some settings - and hey presto all sorted and now matches based on other watches and based on topo & DEM maps.