Uhh, I think I found another broken thing. One of the upgrades removes one of the demon arts from Ironmouse entirely.
So it does. The final tier of the "Crippling Seal" upgrade borks up for some reason. Which is exceedingly odd, because all the other tiers work just fine, and they are coded the exact same way. Hmm...
EDIT: There we go. The final tier of the skill was flagged wrong, so it was getting lost. Easy fix, I'll post a patch in the morning.