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.
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.
One of my biggest bugbears with Microsoft Outlook has always been that its search function is terrible. If you can't find an email then it may as well not exist, and that's been a real problem on a regular basis during my career - particularly latterly when I was in leadership and necessarily lived in my email and calendar.
It's disappointing that Thunderbird has similar issues with such a fundamental function.
Worse though, in Apple Mail I'll search an email address because I got an email earlier in the week and the first thing it'll show me is an email I got from that person 3 years ago with the correct result a few down. I really need a better email client for my phone...
I've been amazed over the years this has never been fixed -- it's very hard not to make jokes about the standard lifestyle of open-source programmers, that they don't consider this a priority (note: that's a joke, I consider myself an open-source programmer. I would hope that's obvious, but someone just bothered to sent me a mean anonymous message)