Didn't get a chance to actually see your arrow, but there is another reason that probably didn't help ya out any. Before the old guy gets hung out to dry.....let me say that cedar is great arrow-wood.....but it does have weaknesses also. Ya didn't say what angle ya hit scapula at but with an high angle striking vector, there is a great deal of lateral stress on the shaft.....it's sort of like giving a shot to a person whose squirming and the needle breaks in the muscle. The cedar shaft Possibly failed from side force and not your head attachment....especially since it broke 1-1/2" behind the head.
Been bow hunting since about 1962, shot a lot of cedar but for the past 15 years I "roll my own" but use straight grained hardwood shafts 3/8" diameter havent broke one since (not on the critter). I have never shot cane primitive arrows, but I believe they would also handle considerable side thrust. Anyways, it might not be your hafting method or material at all.....just might have been a high angle of attack. Just an old guys opinion
rich