It almost seems like some sort of rot. I'm not an expert on wax jambu but could you put a shade cloth over it and cut back on the water?
Don't let the soil dry completely out and become parched though (I mean the soil 6 inches down). For now, make sure it's not wet but try to keep it slightly moist, like a heap of compost.
In fact, soil is a really important thing, as it relates to water retention, and if the soil it's planted in is really bad, you should consider pulling the plant out (carefully, trying not to damage the fine root structure) and filling the area with better soil, loamy dark compost. The mulch you have over the top of the soil is good, helps prevent the soil below from drying out.