Logged! Likely an oversight in the level blueprint, or it might be intended behaviour. I'll ask John.
It looks like it is triggered by digging out where they were intended to dig near the queen. They end up too far away and don't know what to do next.
(Uncertain about it, but it looks like it.)
I had no ants near the digger group, but their last digging target tile was cleared. They started waiting about.
Edit: repeatable
As soon as the digger ants stop digging they become idle. Normally that happens near the player's queen, but when it doesn't the ants just idle about.