Mail can’t find mail since iOS 18

This “some results may not appear” notice keeps coming up whenever I search Mail since updating to iOS 18. It’s been many weeks now (months?) since updating and it’s still only able to find recent emails and I’m not confident that it’s finding all recent emails. It can’t find any emails from more than a few years ago. Mail never had this problem before the update.

Any thoughts on what’s going on?