Plumeria trees are deciduous, meaning they drop their leaves in winter. But, you should also be watering a lot less. The bottom of the stem, at the soil line, looks water logged. That's a bad thing.
I haven't grown Plumeria for years but, there are quite a few members who do. I will give one of them a shout out to see if he can help.
@drdawg, can you help, please?
