Not only is the UX a horror, its results rely on all sorts of technicalities. I, as a software engineer, can understand that a mail has to be downloaded in entirety locally in order to have it indexed and then show up in the results. And I understand that sub-sub directories aren't really a thing in IMAP, so searching this-dir-and-everything below is hard/impossible and so on.
But mostly the search algorithms are poor. So much so, that I often rely on (rip)grep to find mails. Or in a few occasions wrote a quick bash/python horror to push all my mail into a meilisearch instance and then use this search engine to get the lists and filters that I would expect thunderbird to have.
Yes. It's open source. So "go fix it" is a proper reply. But that doesn't make a complaint about the state of the search feature invalid.
Perhaps there could be an option to disable stemming completely from the inverted index, which would be probably easier to implement than a post-search filter (which in itself doesn't sound very complicated..).
But of course, it's open source, anyone could contribute :D.