I am having a very similar problem right now. I'm 100% on the main story and reached 90.3% total completion and throughout the entire game i did not know about these Hunting Requests. Obviously now i know about them I've searched frantically to try and find one but failed miserably. I figured that if i were to hunt the legendary animals, and some normal ones too, i may trigger the requests to pop up, but still having no such luck. i personally think this may be a glitch, i cannot see this being missable as that would void the 100% trophy.
Hopefully Rockstar will release a patch soon enough, my guess it would be around the time Red Dead Online gets released later this month.
But if anyone has any suggestions i am open to them, i have barely progressed since i found this out.