ToME: the Tales of Maj'Eyal

So after repelling harkor'zun in to a wall I decided to dig him out. However the squares between us are mountain chain & not marked diggable. However there's a small gap to the right that is rocky mountain & is diggable. When I start digging the squares adjacent to the dig point change from mountain chain to rocky mountain & I can now dig through them.

I'm wondering if this is intended behaviour or just a side effect of how the engine draws terrain.

Yeah it's annoying when trying to dig out something, I'm pretty sure it was different first half of the year, but this behavior is going on for some time. I think it's mostly for stopping some spells from erasing large chunk of walls in the area, potentially clearing all of mountains, yet to provide a way to dig through big enough missives.