Hard to tell. You think the arrow penetrated 8 inches, I'm thinking not. The deer running with the arrow stuck in its shoulder, will eventually stop and try to pull it out. Probably didn't pop out, it most likely was yanked out by the deer. I don't see where the fixed/mechanical debate is coming in here. I really don't. Was there meat 8 inches down the arrow, or just blood? If it was just blood, no way to really know how deep it penetrated. You didn't do anything wrong. Bad hit. It happens. With fixed or mechanical. Get back out there, make a better shot. Good luck tomorrow!