Why are my plants dying like this

First pic is was a loricata that I got in December. I seemed happy after it got here, it flowered a lot but after a few months the base got dark and eventually turned completely black. I tried to chop and prop and that did not work. There were 4 plants in the pot and eventually they all did the same thing. The second pick a Walmart mammillaris that I've have about 6 weeks. Same thing, seemed happy and was flowering. Looked fine yesterday and this morning it looks like this.

What could I be doing wrong? These are indoors under lights. I treated the loricata like a cactus and I thought it died due to too much heat and not enough water. The mammillaris has been in a cooler spot and getting a bit more water and a bit less light. I'm using a gritty soil mix with about 30% coco coir added to retain some moisture.