I did a quick look through the permissions, and didn't find anything specifically for kick/ban reasons.
Have you (or your moderators) tried using "ulx ban" in console and seeing if the autocomplete works there too?
Yes, they only get to type the {reason} string, autocomplete with the pre-set reasons do not work for them, but for me!
I removed ulx ban, as I do prefer using sourcebans for banning.
EDIT:
When I restrict the string argument to the reasons they get the dropdown menu, but of course typing any string is restricted, so they can only select the whitelisted strings, of course.