I've seen this on several edits, e.g. http://musicbrainz.org/edit/15188477 which is generating much confusion. This is especially a problem when adding a release that has the same name as an existing release. As soon as you type in this field it starts searching and makes it difficult to NOT select a release group. Moreso, once it's selected it's impossible to clear it to make it add a new release group - it always stays green for me (FFX7) and maintains the link to the earlier selected RG - even if I type random gibberish into the field that does not match anything in the search.
It's closely related to
MBS-2951, but addressing this in the short-term seems sensible.