> I don't see any "bad" behaviour with 25.0.94 or 24.5

This is good.)

> though you didn't explain what "bad" means. As far I can see, all trailing
whitespace gets deleted as expected.

For me it deletes not only trailing whitespace, but also a region from a
trailing whitespace to the '/' character(See attachment).
May be it's time for me to update emacs.