Lack of nutrients will not cause a section or single branch to die off. The effect would be more evident as a widespread discoloration. Same with a water issue. Have you checked to see if that branch was damaged some how?

@gardengal48 (PNW Z8/9) I looked and I couldn't find anything like damage to a stem. But there were five or six branches that were dead or dying, and I cut those off. All of the die off was in the same vicinity, so maybe there is a common root area that fed all of those branches and it was damaged?