Kind of a shot in the dark, here, but you don't happen to have any digestive issues, do you?
Undiagnosed or otherwise unmanaged conditions can increasingly compromise the body's ability to absorb nutrients, resulting in malnutrition problems like osteoporosis, weight loss, etc. I know with celiac disease in particular, the ability to digest milk arises early on, to the extent that it people with it can get misdiagnosed as being lactose intolerant, when they're actually experiencing a result of the autoimmune damage to the small intestine.
And, of course, inability to digest dairy products can greatly reduce absorption of calcium and other minerals that make for strong bones....
My current treatment is Rinvoq 15 mg only. It is a selective JAKi and does not have known effect on bones. I supplement with Vit D (20000 UI weekly) and Omega 3.
I had a few corticosteroid injections (3-4 in the right shoulder over the course of 7 years and one in the ankle), which I know can weaken bones.
I took NSAID daily for 5 years (Celecoxib), it was messing up with my guts and I had loose and frequent (2-3 daily) bowel movements, with a struggle to put on any weight. I stopped a year ago as Rinvoq then was good enough on its own to stabilize my inflammation and did not need NSAID anymore. Since then I gained weight with much less struggle.
No other digestive issues, I am not celiac nor lactose intolerant. I had a colonoscopy last year and guts look healthy.
As an extra clue on nutrients: I have increased my dairy intake since I broke the bone and it seems to heal really well (I have not done a control X ray yet but swelling and pain are at zero, I can feel the callus at the fracture site, tolerance over active movement increase). A malnutrition issue would also slow healing.
One more thing. spiral fracture mid shaft of the humerus are a very common injury when arm wrestling, a search of the litterature or experience will show this is not rare. The humerus weakest tensile strength is via shear/torque stress. The break position in arm wrestling could be similar to a bad squat position: What is the break arm position in arm wrestling (please use pictures)? How can I arm wrestle safely without getting hurt? - Quora
Most likely I had a freak incident due to a fucked up grip and bar position rather than some undiagnosed medical condition. I will get a Dexa scan. If it is normal then end of. Ig it shows low bone density then will look at testosterone level, calcium in urine and possibly other tests.