question about terrain & digging

[wobbly]

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.

[Sayan]

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.