search problems

Mac Mail doesn't return complete results when I run a Subject search.

I send about 3-4 emails each month using the same subject line each time - Details for our meeting. When I run a search, Mac Mail returns everything correctly, until about a year back, then skips all qualifying emails until 2018. If I manually scroll back in the account to find and open a qualifying email in the missing period, that email then will appear when I run the search again.

What's the deal?