Are you keeping your neck and head covered? Majority of your body heat escapes from your head and your blood gets cold then circulates through your body and already cold fingers (same thing happens when you're hot and put your feet in cold water - cools you quickly).
I wear a neck gaiter that I can pull over my ears, then a beanie over my head, plus keep my jacket hood on. I keep my hands in my pockets with hand warmers and wear Merino Wool Liner Gloves. This works for me and I have along skinny fingers that freeze easily. Also, I put the hot hands together in one pocket on my ride to my spot and they warm each other and get super hot. Some days I use two hot hands for each pocket.
Also, are you sweating on your way in and after climbing the tree? It may seem just your hands are cold, but your hands and feet will feel cold first if your overall body temp goes down... If so, you may need to keep your heavy jacket off, wear a ball cap, etc. Then suit up after getting in the tree (jacket should be easy to put on in a saddle).
Don't google any medical ailments - 'cold hands' isn't as good of click bait as 'finger cancer' or whatever. The internet didn't go to med school for 4 years and then practice medicine for years after

.