Did you spin the Muzzy? Just a thought that you could have one that is not right. I have shot the Muzzy heads for many years and only had a couple that did this exact thing and they were the 125's. I even think they spun ok, but never shot right. I have one on my bench now I have tried to get to shoot many times.
I think the whole thing about a broad head stiffening the dynamic spine or weakening it depends on the head. They are not all the same. I think 3:1 vented are longer and move weight out, but non vented has more weight at the wide part of the blade so about the same as field points for me. Two blade heads are also different and not always all that long. Seems like the buzz cut is a little shorter. The Muzzy also has an aluminum body, vented, and steel tip. Could be it moves enough weight forward to cause this.
I am shooting the 145 Elite. My tune is a little weak, and I thought the head wouldn’t work out. A 125 would be about right, but the 145’s fly great. The VPA would be my choice as it is non vented, has more weight back and is not all that long. Should shoot like your field points.