Wrong behavior of MuseScore launcher in Gnome Shell
I have noticed that the executable of MuseScore in Gnome Shell opens a new window called "mscore.real" and thus acts as a simple launcher, not as a dock icon. It happens with every dock-like applications in Linux, except the Unity Launcher, which had a workaround for solving the question, but this bug only affects MuseScore.
Can you file a bug report in the ubuntu bug tracker ? https://bugs.launchpad.net/ubuntu/+source/mscore
Are you still encountering problems?
This is a bug downstream introduced by the pakaging of MuseScore on Debian/Ubuntu. I filed a bug downstream.