Occasionally in the water mine an armulite will generate below ground level. I can only see the top of their shell and they are unable to move. They can still attack my character but I cannot reach them with the hammer. Typically it is only one of the many armulites that this happens to.
I have not been able to intentionally replicate this but have seen it happen maybe about five times now.
I’m playing in the X box series X through game pass (build version 1.0-946).