I wonder. Have you noticed in videos etc how pruning roses varies. I still practice the method I learned years ago. Cut at a slant with the lower end of the cut being about opposite a prospective new bud. Nowadays I find that even rose nurseries etc, simply cut straight across the stem. Now this brings me to another point. OK. You visit the garden center and choose a potted rose. More than likely it has been cut back. Here you have a rose bush in a pot cit back to say six inches high. In no time at all, you notice new growth. Gradually growth increases but hey! what's wrong with those two shoots. Obviously the remaining evidence shows that from the budding stage. This rose has grown, now you have a plant with some new growth and oops! stone dead twigs. OK please. I do happen to know about various diseases etc that afflict plants. I have the whatever as being a plant pathologist. IMHO I think, believe that many cases of this kind of die back is due solely to hap-hazzard cutting back. Don't worry says the guy with the sec's in hand. Just chop it off. Who cares. The Garden center gives a three/five year warranty.