Transmuation immunity

I had a player use the Corrosive Body spell figt against some Mithral Golems with the anti-magic ability that gives them immunity to the transmutation spells and some healing. I ruled that the acid damage splashes(from the player being hit) would be healing for the golem and they were not happy about that.

After the game I was looking through the spells with the Transmutarion tag and there were like 3 spells top that can actually damage the golem. I feel like I made the right call, but would like to ask the hivemind about it as well in case it comes up for other players