A little research and I see that what I thought could be a Diaeresis is in fact a Diacritical mark in the Thaana Script used to write the language of the Maldives, which is what MV Boli was designed for.
It's used above or below the basic vowel symbol to indicate either 'aa' or 'ee' ...
View attachment 141837
When I awoke this morning there is no way I could have thought that I'd be researching a language/writing system that I'd never even heard of !!
Every day is a school day
I suggest that your copy of MV Boli is one that has the Thaana characters in but that still doesn't explain why you can't edit or why Office selects it. . . . .