Glad you asked!
When you change a Proposed Merge to an Unmerged Match, it puts the merge on hold. It does not put the merge in line for research. It does not reject the merge. It just puts it on indefinite Hold until someone discovers it!
If you are not the Profile Manager, but you think that a merge needs more research before it is completed, and YOU are unwilling or unable to do the research, please do not change the Proposed Merge to an Unmatched Merge. Let the Profile Managers do that. This is especially true if there is no question of the two profiles are duplicates but the data needs to be ironed out.
Also, everyone should check, under Pending Merges, the Unmatched Merges that they have initiated. They are just sitting there in limbo waiting for someone to work on them, and YOU are the one who started it. It may be time for you to start researching those so the merges can be completed.