Previous bug reports seem to reference specific ones but right now it seems that most (or all?) of them simply do not work. You interact with the NPC, the dialogue happens and then the NPC burrows and the things to mine don’t spawn. You do get a progress bar, but with no way to fill it.
Attempted fixes: Leaving the zone and returning, relogging (both with and without having the quest active), dying and reviving. None of these things worked.
Referencing this thread from the general forums:
Also this one in the bug report forums that only addresses one specific excavation, but seems relevant: