Hello cherryblossoms
Glad to see another San Joaquin country member here!
Something I learned with my plumeria attempts, if rooting a cutting, got to keep it in part shade for awhile till it starts to develop leaves. Our intense hot sun and extremely dry conditions is too much for it. It can take our furnace temps, but the sun is too intense. And got to wait till it shows new leaf claws before you water or at least one fully grown leaf, otherwise, it will just do that bottom rotting. I only water at the beginning when I plant it and leave it alone patiently. Once a new leaf is growing, then I can water freely, at times every other day here, it is just too dismally dry in our areas. Is there another plant or tree where you can put your container to help and shade it for now?.
I would do as Hetty suggested and cut off that portion, let the upper part callous before replanting again. Our window to root them is really slim here. Our overnights are getting cooler.
