Can you rule out that the arrow went where you intended and when it hit the elk it went in and hit the onside leg bone or the offside leg bone and the impact separated the nock from the arrow?
Hitting bone with an arrow or with a bullet can at times produce a very loud smack noise. I hit one with a crossbow and heavy arrow last fall that made a racket like that and then the deer ran. Turned out it did not go all the way through, the deer did not bleed a drop for almost 50 yards and then but a single drop. The deer went down another 10 yards further and everything within 10 feet of it was red. Shot a fawn once with a muzzle loader at ~ 50 yards. Sounded like a turd hitting the sidewalk from 30 stories up. The damn fawn not only didn't go down at the shot, but managed ~60 yards with no blood after the first 3 jumps. Shot through the base of both scapulae about an inch up from the ball joint. Red soup lungs and destroyed 2/3 of the heart.
