Hi there

Plumeria from Hawaii. Recently had two branches, one branch started rotting in the middle, I had a small piece I can hopefully save and root.

3 wks later, the 2nd branch is rotting the same way. Sadly I had to get a knife and try to cut out the rot. Also, the base of the stem nearest to the soil line is hard and woody, with the green flesh kind of falling off.
It’s partially on a heat-mat with a grow light shining on top.

I watered the other day, and it seems like this may have accelerated the rotting. Can anything help halt the rotting? Cinnamon or peroxide?

Please assist