If you deconstruct or dig a block that a grubgrub is pathing to before it hits it, it will freeze in a position and claim to be "unreachable".
I did this over a chasm where the grubgrub should have fallen. Once I rebuilt a tile there it was able to move again.
If I can reproduce it I will edit in a save.
Watch grubgrub pathing by clicking on one.
Destroy (dig/deconstruct) the block it is pathing to right before it gets there.
Grubgrub will be frozen until the block is rebuilt.