As visible in the attached screenshot, i can see through it and i can inspect it. However, my shots pass through it, but i can't move into it.
That phase wall is just completely invincible (phase generator excluded).
This happened after i destroyed the Behemoth standing on the four visible phase walls by aiming at the same tile as the bugged phase wall.
I remembered to make a copy of the save file, so it's also attached in case it's useful.
