I'm assuming it's in a container? If so, how large is it? I'd be interested to know how old the tree is, and if it's borne any fruit yet? Is it getting as much sunlight as before going into the greenhouse? Is your greenhouse tall enough to warrant planting it inside and leaving it there year round? Also, papaya is a very heavy feeder; are you giving it plenty?

I've not had much trouble with my papayas in the greenhouse so far, they've dropped a few leaves but all the ones they dropped were at the bottom, not the top. I grew mine from seeds last spring, so I'm not an expert. The ones in the greenhouse are in 4" pots and very rootbound. The ones in the basement under the grow light look better but they're potted bigger into one gallons and about 2 feet tall.
I've been using 200 ppm of 20-10-10 about once every 2 weeks in the greenhouse during the winter. 20-20-20 pretty much every watering in the summer.
Maybe you should pull one out of the pots if feasible and see what color the roots are?

Mine is in a large container about 3 ft wide 3 ft deep and it is around 4 ft in a 12 ft tall greenhouse and getting daily nutrients like fish emulsion and other supplements. It did try flower but never flowered or borne any fruit it is a bisexual and it is around one year old. It is getting the same amount of sunlight as before. The roots look healthy and white (I just dig around 5 in. down.)

I'm thinking that if it's a year old, it should have fruit on it by now. You said you fertilize with fish emulsion. That's a pretty good fertilizer, but I don't think it's nitrogen content is anywhere near strong enough. Papaya thrives on high nitrogen fertilizer. I use straigt fresh chicken litter in large quantities and my plants "eat it up."

Did you check for any spider mites on the bottom side of the leaves? Sometimes they will dry out leaves and cause them to drop.

I'll check for spider mites and I don't have any chickens to have fresh chicken litter aka poop.

Is it sitting on the floor of the greenhouse? maybe the roots are staying too cool. You could put it up on a pallet or etc so that it's not on the ground directly.
I'd probably start giving it stronger fertilizer too. The days are starting to lengthen and greenhouses are starting to warm up. Maybe you could use the fish emulsion and just mix it a little stronger?

A stronger concentration of fish emulsion is still going to be too weak in nitrogen. And nitrogen is what papaya needs a lot of.