Quote Originally Posted by Dork_Forge View Post
Plausible, but shouldn't be assumed, after all the PCs in question may have never come across animated objects before or just never learned this weakness.
My point is that it's definitely not in the "no possible way the PCs could know without cheating" category. It's not really any more esoteric than knowing that Strahd presumably has a Legendary Action to let him move without taking opportunity attacks. (I'm AFB but I think he's just like a normal vampire, plus spells and lair actions but with regular legendaries.) This thing is a bigger, tougher animated armor, that's all. For all we know they cover this in standard Arcana 302: Investiture and Intent.

(On the other hand, there is no wizard so maybe no one in the party has ever studied Arcana.)