serious stuff
If the rope was set at 50 feet and at a 45 degree angle and the tree was 70 feet tall. Whoa. First of all, I hope you had some length of rope and pulley set-up that put you at least 1.5 tree lengths away from this pull point under the trees fall.
The key to this event is not the plunge cut. It is the amount of force applied to counter the lean and the other techniques needed. If the final product, the back-cut, is the same.
Once either the strap is severed or the standard back-cut is finished. No difference should be encountered with either technique if it is the same cut and an appropriate tool such as rope pull or wedges was used to prevent pinching.
The method(s) and other techniques that allow you to conquer the back lean are the keys.
I think you might want to look at what you did as being actually correct. Too much force, in this case pulling too hard on the rope, is an error that could cause a barber chair. Perhaps you should view the release of the strap, a stable tree, then applying the necessary pull, as exactly what you should have done.
(Caveat: dead wood, if fairly dry, is much less likely to barber chair than green wood.)
Items that may have affected this fall:
1) Hinge wood width, too much could have been a problem, especially in a strong wood like oak.
2) A back-cut too high in relation to the apex or corner of the face.
3) A face that was very shallow, i.e. it only went into the tree 1/5th of the way.
I suspect that with a plunge cut advocate as you appear to be describing, that #3 above would be the most common error in those cutters. If there was an error at all. Remember that the face cut needs to be not just adequately high and open, but also adequately deep to provide a release to allow the tree to fall. A face cut that is too shallow can create unnecessary resistance.
An alternate method that loggers could use is applying the force called wedges.
May I suggest a detente' between loggers and arborists. A more perfect world where the arborists can use both ropes, wedges, tapered hinges and massive insurance policies.
By the way:
What was the trees diameter?
Just how dead was it? Was the wood at the stump dry too?
Did the arborists state any concerns or look sheepish?