That's a new variant on an old bug.
Usually you can shake it loose by removing the suggested merge and starting over. When no suggested merge shows up, it's worse, of course.
Now, I tried it and it looked like I would have been able to suggest the merge. I don't like suggesting merges where I don't know the sources, so I put it as an unmerged match instead.
I was thinking that you could now remove the match and then suggest the merge again, hoping it will work this time.