Lyrics entry mode: too many non-indispensable enabled actions may result in un-typable characters.
This is probably midway between a bug and a feature request.
See this thread for more details and discussion.
The centre point is:
If an action is mapped (either by default or by customisation) to an alphanumeric character short-cut and it is enabled in a textual entry mode, that character cannot be entered as text in that entry mode. Then, in textual entry modes, enabled actions should be kept at the minimum indispensable to reduce this possibility of conflicts as much as possible.
Of the three textual entry modes currently known to MuseScore (Lyrics, Fig. bass/Chord names, general text), Lyrics has by far the greatest number of enabled actions, many of then not really needed in Lyrics entry mode.
While the default short-cuts are carefully crafted to avoid this type of conflicts, the probability of a user customizing his short-cuts to simple alphanumeric characters without suspecting that this is going to prevent the entry of characters in lyrics is not marginal and almost qualifies as a bug.