We do have some challenges correctly tagged with regular-expression+code-golf that don't count as regex-golf in the sense described in the tag wiki:
"Regex golf is a programming challenge involving two lists of items. The goal is to create a regex that matches one list without matching anything in the second."
Some past challenges that are code golf and do involve regexps, but don't fall under the definition above (even in a loose sense), include:
"Write the shortest function that takes one string, and returns a regular expression that matches anything EXCEPT the input string, in parenthesis."
"In this task you have to write a program that reads a regular expression and generates another program that outputs whether an input string is accepted by that regular expression."
"Suppose you have existing regexes you wish to compile using one of the submissions to Compile Regexes. In as few characters as possible, write a program to make that work:"
All these are code golf challenges where the input or output of the golfed code, rather than the code itself, is a regular expression. So, IMO, we could meaningfully declare that regex-golf should be used for code golf challenges where the code being scored is a regexp, while regular-expression would be reserved for challenges involving manipulation of regexps.
On the other hand, we could also equally well decide that this distinction is not a useful one to make, and that regex-golf should be replaced with regular-expression+code-golf. In fact, I'm personally somewhat inclined in this direction myself, even if it does make the regular-expression tag slightly more ambiguous.
(In any case, the same ambiguity arises also with the other language tags, which can mean either "this challenge is restricted to language X" or "this challenge is about parsing / interpreting / generating language X, but you can use any language to do that". Most of those languages don't have separate tags for these two purposes, and I don't think adding such tags would really be desirable, either.)