A personal bug !
Using a BÉPO keyboard I encounter a bug, which I am perhaps the only one to experiment.
The BÉPO underscore key is AltGr SPACE, but it does not launch the expected result in Mscore.
Yet last year I found that modifying the (linux mint) /usr/share/X11/xkb/symbols/fr file I could assign underscore to AltGr"8" key and it is then recognized by MuseScore.
However the version upgrading of Mint smashed my modification.
I made it again.....and moved it in a safe place within my home/.config/MuseScore and placed a link named fr at /usr/share/X11/xkb/symbols/fr
I just write this topic in case another BÉPOist is also using Musescore