Believe this problem has probably always existed. I saw it in our 2.7 instance and now in 2.9. I didn't find any open Jiras for this specific problem.
Sorting by subject does not sort properly if a number follows the text and for one-digit numbers there is no leading 0. For example, Text 01, Text 02, Text 10, and Text 20 all sort correctly, but Text 1, Text 2, Text 10 and Text 20 do not.