It seems like the report is about files with whitespace before the dot of the file extension, whereas the linked documentation describes that no whitespace is allowed after the file extension.
It seems that one of the latest changes made it impossible to create filenames with whitespace before the dot of the file extension.
The Preview-Button shows the correct/expected result, but the final filename seems to get "trimmed":
C:\Temp\01 - Filename with trailing spaces .mp3
"" ->
"Filename with trailing spaXes .mp3"
also the Preview in the Tag->Tag Convert window:
It doesn't matter if you use %title% or %_filename% or %_filename_ext% in the format string. The trailing spaces between the filename and the dot (before the extension) get trimmed.