not sure if that's something infectious or not. it's not the typical "leaf tip die back" from over-fertilization and/or over-dryness at the root zone but I have seen this spotty pattern in some of my softer-leafed phrags, i.e. besseae hybrids. I cut off the affected part, +- a smear of cinnamon, and cut back on fertilizer. sometimes that does the trick.

and an oldest leaf turning yellow is rarely a cause of worry!