It seems that whenever thorns deals damage to an enemy, it counts as having come from your weapon you are holding, which resets the cooldown timer and makes it so your attacks count as minimum-damage swings if you hit right after applying thorns damage. This seems to be a bug, as it does not show up on the attack indicator and is not documented anywhere in the game or online. I made sure of this after extensive testing with two sets of armor, and further evidence is corroborated by the game showing the message "<player> was killed by <item> trying to hurt <player/mob>", which suggests the item is applying the damage or plays a role, accounting for it requiring time to recharge its attack immediately following thorns damage being applied.

I am not sure of the reason I have provided behind this (damage coming from weapon instead of armor), but regardless I am certain of the effect, that thorns messes up the cooldown timer in an unintended way.