That is very odd. Are you just typing “aa” and nothing else? No parentheses?
Does this happen if you type a longer person name? I have a feeling the very short texts, link single or double letters, may not be searched because they would have too many hits.
If I enter @Person1 in the search field and search for @Person1, I get no results.
However, if I just enter @Person and then tap on the suggested @Person1, the search works as usual.