Its hard to say with any certainty what is the cause. The typical reason is related to being too wet or too heavy of a soil mix. Which based on your description was unlikely. There could be other reasons beyond that like a bad cutting (rot was not apparent but was already there), bad cut (not properly callused), and/or seasonal issues (too cool and not during growing season).

Sorry but that may be the best I can say. My preference is to start cuttings in the springtime on a heat mat with natural light. IMO light is less important than warmth.

Plumeras are easy, when you know what the like.

Hot, WELL drained soil, lots lots of light.

So if its rotting its poor drainage y too much water. In nov-may in nature the go with 0 , (zero ) water. Y thet live in rocky sandy mountain sides.