This is the most common question asked in the late fall. Many new plumeria growers just do not know that plumerias go dormant over winter.
But this is the annual, traditional rite of passage into winter, or dormancy. Soon all the leaves turn yellow and will fall off and it will need no more water until new leaves emerge in the spring.
[This is the time of the year when your neighbors see your naked plants and complain about the deer antlers in your front yard!]