A few days ago, I was playing 'Excavators' on hard. During the first attack of mole crickets, I dug a tunnel as in the image (left side tunnel) to go towards the cricket. However, the path for that cricket was different as in the image (middle). I had already marked for excavation. The workers dug the tunnel in such a way that it opened up in the middle of the cricket's path.
But the bug is that exclamation mark in the middle. As John said in the development videos, the exclamation mark is a pawn (pawn is a class in unreal). When the tile or chamber surrounding the pawn is dug, it destroys itself (destroy actor is a function in unreal that deletes itself). It then spawns the creature corresponding to the creature ID. But the problem is that the tile that surrounds the pawn is not yet dug. While other tiles were connected together, they got dug out at once. First, the pawn enters an undug tile and then it dugs it after some time and moves to the next tile. Unfortunately, the worker dug to the path when the spawner was not yet dug the tile it was in.
To fix the bug, probably you need to change the digging system. Maybe instead of having the pawn enter the tile and then dig the tile, have the pawn dig the tile first and then go into the tile. This way the spawner would never be in an undug tile. Just like workers dig, they never go into the tile.
I do not know if there is any other problem that may not let this system work. However, it is just a suggestion for a way to solve the bug. Of course, I am just a beginner in unreal.