Mud golem healed by sneak attack?

My arcane trickster sneak attacked a mud golem with acid splash, and for some unknown reason the mud golem was healed by it, and then also took damage. Is this a bug or is it some special ability that is not written in the monster’s info? I tried to attach a screencap but failed.

I did not find mud golem but i’m pretty sure its the same ability as clay

Any magical attack against a clay golem that deals acid damage heals 1 point of damage for every 3 points of damage it would otherwise deal. If the amount of healing would cause the golem to exceed its full normal hit points, it gains any excess as temporary hit points. A clay golem gets no saving throw against magical attacks that deal acid damage.

I don’t think that’s it. The mud golem was first healed for 48 HP and then damaged for 16 HP. Also, it doesn’t appear in the creature info.
The mud golems in question appeared in the BtSL DLC.

Octavia sneak attack count as elemental(acid) when use with acid splash for example. That can explain the healing part a little bit but not the damage he then take.

Is it intended? That is a other question…