So running into an issue later in dev that this behavior is maybe too unique for the rest of the game so players aren't figuring it out but we don't really want to change it either. Its also the first time you run into this (of like 3 times).

I've wrestled with this a lot. I think this might work.

We do have an obvious orange color when damage happens and different sounds for hits vs no damage.

I think ultimately the problem is we don't have enough enemies that function like this, basically most everything else is hit it anywhere and it is damaged...

I thought that too until watching real players struggle to figure it out when I demoed my game.

Was trying and failing to come up with other ideas than this to make it more obvious. This felt like a funny comment in the spirit of the game itself.