This sounds like a fungal/bacterial issue! This should be corrected as soon as possible. I have a recommendation, and recipe that I use, personally for occasions such as these. The recipe is as follows:
First, you will want to prime the soil with dolomitic lime, then spread wettable sulfur to Kill off any infection in the soil. I Also like to spray the plant itself. Mix a few drops of one or more of these ingredients into warm water with a spoonful of coconut oil: Rosemary oil, lavender oil, neem oil, garlic oil, cinnamon extract/oil and for really bad cases, also add clove oil.
Here is an article that will give you more information on the care of these: https://www.gardeningknowhow.com/ornamental/flowers/plumeria/growing-plumeria.htm