I can't move or dump items that were dropped by Dwarfs interrupted in a task

I had two dwarfs build a wall when they were interrupted by the werefox Iño Enemtuthru. Apparently a large fox twisted into a humanoid form is enough to drop everything and start running.

The problem is I can't get rid of those two dropped granite blocks.. They are marked for dumping, but noone has picked them up in a long time and now they are blocking further constructions. What can I do to convince the dwarfs to pick them up? Is it a bug or am I missing something?