I'm looking at Options > Index > Sorting. Under Sort Method, the second field is Ignored Characters. I've worked with this many times before because I often have to add the half circles that many authors use for ayn (ʿ) and hamza (ʾ). (And when I do so Sky changes the first few characters to something weird, so I have a template that I use with these characters in the field. I spoke about this in my webinar on using diacritics and wrote about it in my article on the same topic.)
The accent grave was there by default, but it is not getting ignored.
I see now that the fifth field under Sort Method is Symbols. I've never bothered with this before, but it seems that the accent grave is there also. Perhaps if I remove it from Symbols things will work? . . . . . .
Nope, that didn't work. (And the same weirdness occurred when I copied it back into the field.)
Still hoping someone can figure this out.