i want to add that my server does the same thing when restarting. the tunnel bore inventory is wiped even though the world anchor is working. so it seems that when the tunnel bore is not loaded in the world, it's inventory is "wiped" kinda troublesome since my server has automatic restarts xD
i also noticed that when i dug with it, suddenly a bunch of suspicios entities appeared: coal coke, rails, gravel, that stuff that should have been in the tunnelbore. you cannot pick it up though. so my idea is that the inventory gets emptied into the world in a way tha makes entiti duplicated along the digging path.
anyway, are there anyone else who can test this to see if the same things happen on their server? i will probably end trying it on a local one for my self.