Edit: It’s currently also showing that 7 our of the 10 elite difficulties are waiting on DPS. That doesn’t sound right at all. I think the whole groupfinder system is borked at the moment. This does happen with some regularity.
I think that the group finder displays missing DPS when a queue needs all roles (which seems pretty common in most elite queues).
I was queued as tank and another in my group as healer, so your theory isn’t relevant here.