I am finding tunneling to be very problematic/painful/frustrating.
Both in digging new tunnels and re-digging parts that filled in, the height of the new segment seems variable such that I can't walk into it. I have had the ceiling for the next segment be:
2) okay, but you get hung up on the ledge/intersection so you have 'wiggle' the toon back and forth to get past
3) impassible because the ceiling height of the next segment is anywhere from waste high up to 'hitting your head enough to stop you'
I have tried 'dig level' followed by 'dig down' in the same square with results variance between ok (rare case) to it not even allowing me to do the 'dig down' on that segment.
Bottom line: there seem to be any consistency on either original (first time) digs and repair digs. Like I noted earlier: sometimes it lets me dig an open tunnel square and I can go into pit mine mode, and other times I can't do anything. It also doesn't let me mine out the overhang to allow myself passage ('dig level' nor 'dig up'). Very much back to this game's interface being extremely non-intuitive.