ToME: the Tales of Maj'Eyal

Unless I'm missing something, I've encountered a bug in a Hidden Vault zone linked to HP(1).

In the screenshot, in the top left corner, there's the stairs to HP(1).

The two red circles show where two of the usual "rock locking a vault" were found. The top left one revealed the vault I explorder, the second one, in the bottom right corner, leads to nothing. I clicked, it disappeared. That's it.
I've tried digging every single wall section in the level, both with magic dig and pickaxe dig, just in case, but none were diggable.

Is there supposed to be something accessible after the second "vault locking rock" in the bottom left? If so, then I can't access it, if not, then the bug might be along the lines of the random level generator attempting creation of a vault within a vault.

Note though that in the middle of the vault (connecting the two "U") there's a digged section that joins what would otherwise be a symmetric layout of, possibly, two "U" shaped vaults that mirror each other. This interpretation leads to the idea that there is supposed to be something on the right of the bottom right red circle, but that it became unreachable.
Since this is my first time reaching this point and Google hasn't given me answers, I just want to check I'm not missing on something. Thanks!

I think that's how it works. These hidden vaults reuse vault layouts that you can find on normal maps, thus two entrance rocks.
Here is a vault I encountered that has two entrance rocks that lead nowhere. Level change stairs spawned inside the vault itself.