The search bugs in Mail are absolutely real. They might not affect every user but I've had major issues on both macOS and iOS mail, and eventually gave up and switched to Gmail.

Have you checked if it’s the fault of your mail providers? The search function queries the remote mail servers in addition to the local cache. If the mail servers return garbage you get garbage results.

As I used to ask field teams and PMs back when I was a SWE - provide the steps to reproduce the bug as I can't seem to recreate it.