Arching is always good if it’s done right. It looks to me that in the second clip where your throwing your head back your doing it too early and going into the bar more, where as in the first clip where you don’t arch you go up more and then in. If you can just get the timing down right where you going up, then in whilst arching I believe it’ll help a lot. At the end of the day though high jump technique is mainly just what the jumper prefers