It's because SE uses a stupid priority system for the adventurer in need.

It checks for tanks firsts before checking on the other roles.

If there is a single group that still needs a tank, then the tank role is the adventurer in need. Completely ignoring the fact that that the same group may also needs a healer, and so does a dozen other groups.

td:lr 1 missing tank vs 100 missing healers = tank in need.

So you're waiting 5 minutes as a tank, because there are no healers queuing either.