I'm mainly putting this here to have a link to stick in the Bugfinder to hopefully get this bug fixed.

For anyone interested, the bug is currently located somewhere just below the stairs in the center of the volcano, and the end result of it is not being able to build stairs directly below, presumably because something somewhere is flagged as inaccessible.

I have not used DFHack-liquids, so that is not the cause of this problem. Submerging the area in lava does not seem to help, nor does building constructions to approach the site from a different angle - in any case, I find it hard enough to continue around the bug's location just to go down the insides of the volcano, let alone poke around looking for exactly what it is causing this problem.