It appears the Fire Elementals in Furnace cannot get off islands of non-lava land. They have the SWIM flag set, but never cross the lava.
Example at coordinates in SI: 1381, 2614
This is with the latest build, but may have been an issue for a bit longer.
I fear this may have never worked in Exult. I could only test with a built from 2010 but back then it was already not working
In monstinf.h, I see we have flags from saved games for 'swim', 'fly', etc. Maybe there's one for walking on lava.
This was a difference in how Exult handles "swim" versus how the originals did. Seems like the original lets swim move into any "solid" floor tile, and requires it to be a water tile only if you don't also have the walk or fly bits set.
Fixed in master.