I don't know why you're getting downvoted for this. That seems like a pretty frustrating bug when generalised to other word stems. It's also pretty standard to prioritise exact matches when ordering search results so, again, frustrating.

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.