Hey NCommander,
I went ahead and tried your save out in the arena, and the GCS webbed things just fine there. Then I let the guy out on your save, but instead of wreaking webby destruction he ignored all the dwarves in the immediate area and started just running up and down your central staircase. He did attack and kill a single cavy boar, but then resumed his stairmaster training. He didn't attack any of the numerous dwarves and cats that he passed on the stairs, and they didn't seem to be afraid of him, either. Nothing in the error logs. I tried opening up a passage to let the GCS out into the caverns a different way, but he didn't seem interested in that, either.
When I ordered the militia to attack him specifically he did finally engage, but not until he was attacked. He never shot web in the rest of his life.
I'm 99% sure this isn't due to something in the raws, since GCSes using your save's raws shoot web just fine in arena. If I had to guess, I suspect it's something to do with the bug where NPC special abilities sometimes get "stuck." I've only ever seen it for "
roll up for self-defense"-type abilities before, though.
I recommend you make a bug ticket in
Mantis. Be sure to link the save on dffd in your ticket
.