I don't grow these but maybe you should move it to dappled light? Prehaps the roots were too wet for too long and compromised (but was not evident to you at repotting time). Maybe your mix is drying out faster than you think in that heat? Just thinking out loud. Good luck.
Oh, I'm sure you know, but the sap is caustic do not get it near your face. I much prefer Bottle Cactus for this reason (and the fact that I like Bottle Cactus).

Here in the Phoenix area, I have to keep mine in a shaded spot in the summer, or they burn. Good news is, they're almost impossible to kill, like most euphorbias. Mine like more frequent watering than a lot of other succulents, maybe because it's so hot here.

Too much sun and heat before they've had a chance to develop a good root system.

First, anytime you repot a plant you should keep it in the shade until it's fully recovered. I'm not saying you need to put it in a closet...bright shade is good, you just don't want direct sun because that will dry out the plant extremely quickly.

Next, you only need to wait a day to let the roots callous after repotting. Then water frequently, based on how turgid the leaves are. For my plants this usually ends up being once/day for a week or two. What's happening during this period is that the plant is growing new roots and the fine root hairs that do the bulk of the water extraction from the gritty mix particles.

Once the leaves can stay turgid for a day I start spacing out the watering, again based on leaf turgidity, and I gradually increase the sun exposure starting with dappled shade for a week. Then a week of an hour or two of morning sun, then a week of half-day sun, and so on. Btw just because the plant *can* eventually take full sun doesn't mean that you must give it that. A half day will probably be fine too. Temps over 90F are tough for most succulents, particularly in pots since the roots get so hot, so I move my potted plants to a spot where they get morning sun only or dappled shade on those days.