Well, your stems do look a little desiccated, the fact that when you cut them you got clean tissue and white sap means that they are seemingly healthy. Perhaps you might need to give it just a tad more water?
When we were in the Cayman Islands plumeria grew naturally there, by the side of the road. No one took care of them. What we thought would be a lush, jungly island with gingers, palms and heliconias everywhere was actually a rather barren nondescript landscape. The only places that had the lush vegetation we expected were resort hotels where the plants were taken care of (at great cost, because fresh water for irrigation is costly).
The plumeria there grew in basically rocky sandy scree as tropical succulents and the only water they got was what they could soak up when it rained. Maybe deep water your plant once a week? Or when you see the leaves wilting.
