I started a treasure map mission (extra wealth omens), which had the objective of killing elder soul spawns, if I remember correctly.
The second to last one was unreachable, apparently inside a piece of wall. The map tileset was I think the necropolis?

I was able to see damage numbers pop up when using annihilation at the position the enemy marker said it was, but it didn't seem to do any actual damage. I never saw a health bar, and even after many hundreds of thousands of damage, I hadn't killed it.

Had to quit to main menu to exit the mission, because I couldn't complete it or use town portal.

Since this bug has been around since the early days, perhaps the objective could be changed to something like "kill 10 elder soul spawns", and then you have 12-13 on the map? That way if a few are unreachable I could still finish the map.

Obviously it would be ideal that they don't spawn in unreachable places, but that seems to be a tricky bug to track down.

Cheers!