Hi!
Just imported a few GPX files an noticed, that the data is not correctly shown in Garmin connect. The same file shows loaded into basecamp shows the correct max height of 2457m, all other datas are also displayed correctly. The same file imported to Garmin connect shows a way lower max altitude and some other corrupted data as well.
I attached one of the files, maybe some has a solution. By the way, i tried the same as tcx with the same result.