Answer: There are both evergreen and deciduous plumerias. The deciduous kinds lose their leaves but the stalk or stem remains. If yours remained evergreen for a year and a half, I'd say yours are evergreen and should not be losing their leaves. Spongy, soft stems indicate a root rot or crown rot due to oversaturated soils. (May have been caused by poor drainage, too much water from the sprinkler, or even several days of rainfall.) I don't know whether the problem is throughout the root system or simply the tops of the plants. You may be able to stop the problems by applying a fungicide, or you may be able to simply cut away the diseased stalks and allow the roots to replace them with healthy new stalks. If they were my plants, I'd carefully dig one up and inspect the roots and crown of the plant. You should be able to tell whether or not the squishy stem is a result of root or crown rot by pressing on the crown and the roots. Cut away all the squishy parts and pot up the remaining parts of the plant in fresh potting soil. The roots may rest for awhile before you see any new growth, but healthy roots will produce new stalks.
Best wishes with your plumeria!
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