Swiftblade merc not moving with Gruthkul's pelt equiped

Bug ID: 3.546.164.185

I just got a new merc, a Swiftblade. When I was equiping him, he can wear a Gruthkul's Pelt. However once I entered a map, he was frozen in place. I know spells are disabled with this armour, but that does not mean he's frozen in place.

After I removed the armour, he was running around and doing his thing again. The rest of the gear I kept on him.