When there is a solid inside a door (bunker/airlock/manual airlock) the RM (robo-miner) won't mine this solid and is blocked by it.
Example: regolith falling into a door won't be mined by a RM. Instead the RM is blocked by the regolith inside the open door.
This behavior doesn't presist through save/load since the element in the door gets deleted on reload. (Another bug to report or intended?)
put solid in door (debug tool or falling regolith/sand)
have working robo miner in range
be awestruck by the robo miners inability