This is actually the same bug as the "robo miners digging infinitely" one, just more general.
There are 4 common instances of this bug happening:
- Two dupes digging a 3+ tile "tower" of sand from below. The tile at height 2 is dug, the tile at height 3 starts falling, the tile at height 1 is dug and immediately re-created with falling tile. Now there's an undiggable tile at height 1.
- Two robo miners try to dig a tower of regolith. Same as above.
- A rocket is embedded in regolith and then starts. A similar situation as in the above can happen.
- A rocket is embedded in regolith, but there's some coal, algae, slime or dirt in the regolith too. The rocket destroys the regolith, but its heat causes one of the materials to change phase in the same physics "tick", creating an undiggable tile.
Attached a screenshot with an example of the last point. This rocket has came back, but failed to destroy that refined carbon tile. You can tell it came back onto the tile because there are items on top of the tile - if it formed after landing, items from landing would be buried inside.
Not attaching a save since the bug fixes itself on save.
Doesn't reproduce reliably due to precise timing. Will eventually happen in any normal game that reaches space age and sends enough rockets. See examples in issue description.