One possible explanation for why your exact match queries aren't working is a peculiar edge case related to the use of hyphens.
If there are two dashes and one letter suffixes in the beginning of a word, such as "AB-A-B", searching for "AB-A-B" will not yield any hits.
The issue occurs when the ending prefix repeats a letter present in the word.
If this isn't the issue you're encountering, please contact the Support team.