Plumeria rubra goes dormant in winter. This is usually in late December, when the leaves will gradually yellow and fall off. Your rooted (hopefully) cutting is likely doing the same. You need to keep from watering it until it leafs out again in the spring.

Can you confirm that it is rooted? How long are the leaves? And can you post a pic of the plant?

If you can put your plant under artificial light set to more than 14 hours, you might be able to keep it from going dormant.

I'm very new to this, so how do I check to make sure if the plumeria is rooted or not? I have four plant total. They all starting to grow leaves but some new baby leaves are turning black and fall off. I took them out of the green house because I have a misting system in there and it get their leaves wet, I thought this might cause the baby leave to turn back and die. Is that true?? That's just my thought. :). Currently their are under some led light with the surrounding temperature around 70 and humidity at 30-40. Please help

The ones with longer leaves look like they are rooted. But they are wanting to go dormant so, so I would def. reduce water. If they retain quite a few leaves, you can water 'just a bit', but a misting system is bad for them. It's OK for them to go dormant. Letting them go dormant also helps prevent a lot of spider mites in winter.

It's normal for them to abort leaves this time of year. They would likely welcome a rest. No water for that one.

What are your lights set at (time)?

I planted about 15 plumeria 7 years ago. The survivors look a lot like yours. They like it pretty dry, so they don't do well on the rainy side where I live. Even in dry Kona, they lose their leaves in the winter.

Dave, my light setting is at 6 am in the morning to 4 pm and then on again at 10 pm to 12 am. I use some led with a lot of red light bulbs but it seen to burn the leaves. So I stop using it and just use regular fluorescent lighting right now.