The situation described by mobius happens because Fencers stop Fencing if there is no pixel at their foot position. Since a Miner tunnel necessarily creates a dip of several pixels, there are a number of frames of the Miner animation from which a Fencer, if assigned at that point, will not be able to continue Fencing.
My thoughts here are that we can probably relax this a bit to grant the Fencer a bit more versatility and assignability. Fencers should be able to adjust to at least one pixel missing at their foot position, in much the same way that Bashers can; what would happen is that the Fencer would would step down into the missing pixel, find solid ground, and continue with an upwards trajectory from there, as usual.
I'm happy to go ahead and implement this change unless there's a significant reason not to.
Thoughts?