%_file_mod_datetime% returns a text string, which contain some characters not to be allowed to be used in the file sytem.
The "Tag-Filename" preview removes such forbidden characters automatically, that's all.
I tend to say it is not a heavy bug.
But there might be a quirk in the sequence of applying the validating rule.
See there ...
Converter Tag - Filename validates part of format string before applying other functions around
DD.20120408.2058.CEST