That is a bizarre bug. There is an even simpler way to reproduce the problem: Have two pics, one with title "a" and the other with title "t". The criterion "Title Contains a" correctly shows just the first picture, but the criterion "Title Contains t" shows both.
Searching with smart collections and filters has numerous bugs, but this one is by far the most surprising. As a geek programmer, I'd love to know what causes this!
Adobe rarely reads this forum -- I suggest you post this in the official Adobe feedback forum, which Adobe does faithfully read (but only infrequently responds).
Works as expected here.
Win 7 x64, LR 5.7.1
"Title contains t" - shows only one image.
This might suggest that it is only Mac related.
Thanks for your feedback. I have posted this problem on the official Adobe Feedback Forum.
Interesting -- the problem also occurs on my Windows 8.1 / LR 5.7.