Preferences/Shortcuts doesn't always save double augmentation dot shortcut

I've been transcribing some old scores that use lots of "double dots", so decided to create a shortcut.
At Edit/Preferences/Shortcuts, I select Double augmentation dot/Define and enter "," (comma). The shortcut works fine for this session, but if MuseScore is exited and re-opened, the shortcut is no longer there. I've done other shortcuts with no problems (mainly Shift+a number, the reverse of Alt+a number to add chord notes higher or lower than the selected note), so I don't think it's operator error.
I found when experimenting that certain double dot shortcuts *will* save properly, such as Ctrl+> or Ctrl+*.
Any ideas? The comma shortcut works best for me, since it's right next to the period. Also, it doesn't seem to be used for anything else.
PC, windows 10, MuseScore 2.02

Thanks,