See http://musicbrainz.org/edit/26203164 for instance. In my opinion, that behaviour is totally intuitive. I've come across a couple of other examples where I couldn't seem to find a certain RG anymore until I realized a stray RG had been merged into it, and it 'inherited' the secondary type which made it slip to the second page or so.

Going one step further for consistency's sake, we should consider not merging RG types at all. In almost every case, a (new-ish) stray RG is being merged into an established RG that already has correct values, so that the opposite case where an established RG takes over the empty value should be a rare occurrence.

The ideal solution would probably be a preview of the outcome of the edit – and an option to edit the value on the fly when necessary.