I finally realized that when you don't install in the standard path "C:\program Files", (Mp3Tag v3.27a x64) the shell integration doesn't work (the Mp3tagShell.msix seems to be "hardcoded"
Would be great if you adapt msix to be able to work for every installed path.
Actually I did a HUGE cleaning (reg, appxpackage,...) and NOW it is working....!!!! Both context menu are working well now... (modern and classic are present now...)
Don't know exactly what happend at the first installation....