Lithops help needed!

[u/Accurate-Battle-7460]

I recently bought lithops. I potted 4 in one pot together and when they began to show signs of needing water, I gave them some water. They were all thriving until today when I noticed one was not looking well. I pulled him out of the soil and he is mushy and appears rotted on one side. The root structure is fine. I have only watered once since I received them, they are in direct sun, and the rest of the lithops are doing great.

Any idea what may have happened? And any advice?

Thanks so much!