Sometimes indexes can get scarmbled. Blame it on windows
Kris is right. Sometimes indexes just get scrambled and need to be cleared.
BTW - Minimal vs. Full really isn't the issue. That refers to whether the field has a word index, a value index, or both. (Either word or value = Minimal; both = Full) The word index is what's used for sorting. Just because a field says "Minimal" doesn't mean it's properly indexed for sorting; it might have the value index and not the word index. (The value index is used for relational joins, for example.)
I guess you could repair the index with re-entering value, after delete and commit it.
you can reset corrupted indexing by turning off indexing for that field ( Make sure all validation is off or it will not disable) Then close DB manager then re open and re enable indexing. Unfortunately Filemaker is susceptible to corrupt indexing on mac and windows.