I've had issues with the thumb knuckle behind the ear - ended up with blood all over my thumbnail. Turns out my hand wants to "pronate" or turn palm-down, and the nail was chewing a hole in my neck while my fingers were torquing the string.
Remedy so far is to extend my thumb up the back of my ear, from there string pull keeps the bottom of my hand in closer and aligns fingers better with the string. That has improved release and consistency -and stopped the bleeding.
A shooting school grad who watched me noted that pulling the extended thumb past my ear meant I was drawing back "off line", answer to that was holding thumb and forefinger together until past the ear. May not be ideal, but I don't want to spook the deer with the scent of blood!