Hello everyone!

First timer here. I have a 3 year old plumeria plant that I have been growing indoors with sun lamps in Washington. It's growth is very slow given the limited sun and warmth WA has here, but its managed to stay alive so well all these years! During the summer, it will be placed outside and have lots of leafy growth. Once fall hits, it's brought inside but will always stayed firm throughout the winter with a handful of leaves still on.

About 2 weeks ago, I noticed the section where the only two branches emerged from became soft (pics below). I was worried for stem rot, but have been given little water (once every 10 days or so) due to winter approaching and assuming dormancy was going to happen. But then, the branches started to shrivel, the softness spread and all the leaves became droopy (usually only the ones furthest from the tips would slowly turn yellow and fall off). Plant continued to go down hill, and I cut one arm off to check for stem rot. Inside I didn't see any brown but I noticed the stem had a partial hollowness. The other arm is also still soft and the main stem near the branch is partial soft. Any idea on what could have causes this? Should I cut the stem until it is no longer soft? I'm mainly interested in saving the plant and don't mind cutting off more if it means the stem will live another day to sprout new branches.

The main base near the soil line is still firm, so I'm not sure if I cut off the arm in vain and my plant is actually fine. I have three other plumeria nearby that are fine, so I have a low suspicious for pests.

(One photo shows all my plumerias, but the one Im asking about is on the right in the leaf patterned pot. That photo was taken two months ago just to show how much it booms during the summer.)

I'd say it is massively dehydrated, possibly some root rot? I would dig it up and check the roots.

I watered the plant and initially improved a lot with firmness returning. But then suddenly, the rest of the leaves fell off. The firmness in the remaining branch began to disappear and spread toward the base. I started picking at the arm and stem rot had developed sadly. I assume that when I cut the first arm off, the open wound exposure allowed bacteria to get in and thats why the rot began in the branches and not the base.

I've cut down the base 3-4 inches until I saw healthy tissue. Hopefully, the stem rot stops at this point. Will new branches form eventually or is this plant a goner? The roots looked fine when I dug it up and the base is still firm.

Might be a good idea to sprinkle some Cinnamon powder on the Cut to dry it out. (If) you've been misting... STOP.

We are heading into Plumeria sleep time as winter ensues some more...just got to keep them dry, no watering. Looking at your media, it really needs to be much grittier than that if you really intend to keep it awake if you can this winter. But, if it does go dormant, while it is dormant this winter, you can improve that, add much more pumice or perlite. So when Spring comes around, the plant will wake up much better with a much more well draining media.

Just to give you an idea, this is how gritty my Plumeria media is:

June 2017 file photo

as you can see, like all my typical succulents, they are all planted likewise in a gritty well draining media. Plumeria on this photo is finally waking up from its winter slumber. Took a long time, really wants warm to hot temps. So in winter, with lighting not sufficient, and temps cooler all over, it will just sleep it out, and wait for late Spring to Summer to wake up again: