Looked healthy?
I don't understand. If the Plumeria was doing pretty good and it started to droop suddenly, that often means either over watering or under watering. The roots die and do not disappear right away. Just because they are fibrous and brown does not necessarily mean that they are alive. They could be damaged from the improper watering.
I am looking at this as a simple equation, the clues that you mention lead me to think of a water issue.
Plumeria are semi tropical or tropical plants. Water, warmth and sunshine are the keys to good growth.
If you want too, I would just cut off the bottom of the plumeria to healthy area. Let it sit for a week or two until it forms a callus. A callus is like a hard, dry scab. Then after the scab forms, place it in a pot, not too large, and wait for it to root. Do not water at all. It should root rather quickly. After you are sure there are new roots you should also see new leaf growth at the top. Lots of leaf growth!! Then you can water a little sparingly. They do not like being too moist or soggy.
