Disappearing roofs/mountain tops

[M]

I found that bug while in an icy game (no harvesting season).
I dug my base in a huge mountain. The base was going well and hydroponics were all set up and doing their job.
I had 2x11x11 rooms with hydroponics and from one of those I dug into the mountain farther to create new rooms for stockpiles.
Once dug I discovered a new unroofed area and suddenly and instantaneously even my 2x11x11 areas have become unroofed (both with mountain roof). The new area was connected by a corridor (10x1) to those 2x11x11 areas.
Building a door in the corridor solved the problem (the farthest regained a mountain roof and the nearest a constructed roof), but I still think this is a bug.

Unfortunately I do not have the game-save but it was in Alpha 9.d

To reproduce this bug dig into a mountain with undiscovered open unroofed areas.
Build a door on the entrance of your cave only so you will have roofs on the next rooms.
Then build rooms without doors (10x10 size, or big at least, mine were 11x11 with L shape colums on angles) near the undiscovered area. Once dug the rooms dig to discover the unroofed area and the bug happens.
The unroofed area can be distant from the rooms (mine was connected by a 10x1 corridor).

I think the bug occurred because the game tought that was an unique room cause there were no doors between the new area and the old one. The room was considered too large and the roof was removed as half of the room was already unroofed.

[Gaesatae]

I've read your post a couple times now and I'm not able to completely understand what happened.

Are you saying that your existing room under the mountain, covered with natural rock roof, became fully unroofed when you discovered the new area?

[Tynan]

I believe you're seeing this issue:

https://ludeon.com/mantis/view.php?id=1637