I just reproduced this with a 2 Ghost Bats being attacked with Missile weapons.

Yeah, confirmed extensively in my plays.

I... kind of like this. It adds a certain strategic element to some fights. Yes, it's a bug that needs to be fixed, but if this were kept in somewhere (Earth Temple?) it might be kind of cool.

I'll admit, I do also kind of like this and can see where it might be useful.

I just had this happen to me and wondered if somehow there hadn't been an empty space there before I killed the ghost bat. Glad to see I'm not just seeing things!

If this was introduced to prevent unwinnable games caused by monsters who pass through walls picking up chaos orbs, it's redundant, since monsters no longer pick up artifacts, including chaos orbs, starting with p1 (issue 735). Also see my proposal regarding that fix (1821). I don't think it was introduced because of the ASB because the problem of the ASB dying on a wall square, undiggable or not, was fixed a very long time ago, in the 0.9.9 gammas: if the ASB dies on a wall square the wall always disappears.

I know this might sound a bit radical and expansive, but why can we see and "hit" monsters that are inside walls?

Poison might still kill those vulnerable to it.

Maybe "Earth" creatures can die in a special way, destroying the wall they occupy? They are 'dead' now, so their 'body' pushes the wall into a crumble.

You are next to the wall, and they have to reach out of the wall to attack you.

Yes. If they can't be damaged while in the wall they can't die in the wall.
Only from poison/bloodloss/mindattacks? and undead are un-affected.

The complication is: My Earth Elemental companion surely should be able to hit a ghost inside the wall?

Maybe ghosts, etc. should not be able to carry items?

There is no reason to complicate things that work perfectly and make perfect sense (despite protestations to the contrary).

I hope you are joking, but just in case: walls disappearing because a spectre is detroyed inside doesn't make sense, if it does to you, could you please explain how you've come to such a conclusion?

One, I think that's a perfectly fine addition, even if it was a bug. Two, surely you realize I was responded to this harebrained scheme to not be able to damage creatures in walls?

I realize that now.

Then simply having 'Earth' creatures destroy the wall when they die and incorporeal creatures not being able to carry items should solve it.

If this bug has no other consequences, I wouldn't mind to see it promoted to feature. Mystery, eeriness, the surprise of a ghostly encounter: to me, the wall disappearance seems a nice addition to the atmosphere of the game.

Considering there is a special message for digging and finding items that a phasing monster after dying inside a wall, I'd think this is a bug. The ASB probably removes the wall tile upon death because the Earth orb is vital to win the game, and TB wanted to avoid frustration if the player's digging tools break/run out of charges? Even then, it still won't drop its corpse if it dies in that fashion.

Humanoids not picking up artifacts doesn't make sense either!

That corpse thing is highly annoying.

Pretty much. This wall removing issue was first mentioned, and brought to the Creator's attention, here.

Wall-ignoring monsters now leave their possessions on a random non-wall square next to them, and destroy walls only if no other solution is possible (I'm not sure if this will ever happen).

Sounds like a great solution.