From my own experience through trial and error I noticed that the rainbow cactus and all fuzzy looking cacti with very dense white hair like needles are very sensitive to overwatering. You cannot follow typical internet instructions on cacti care. If they're getting plenty of afternoon sun you have to neglect them until they look a little wrinkled. It's the same with the echeveria and Split Rock succulents. I don't know why this is. I do not have this problem with the thick needle cacti.