So I just wanted to check if it's intented to be able to infinitely duplicate unenchanted sentient armour sets just by taking them off.

I mean, I like it because it allows me to decorate my base with a bunch of sentient statues, but I'm not sure if it's intentional?

It's not necessarily intended, more like an unintended consequence. If you were to give the armour to a friend that doesn't have enough Will it will just disappear on the next hit anyway, so no actual exploit with just Vanilla + BM.