I’m just spit balling here, so ignore me if you see fit. Won’t hurt my feeling. (I only have one left.)
Can’t tell from the pic, but is the work piece supported by a live center or steady rest? If so, could the work piece be “unseating” itself as the cut proceeds? Tailstock slipping? Dead center with insufficient lube? Does the transition in the finish always happen at the same location in carriage travel?
I don’t understand the “too shallow of a cut” theory. My bench lathe isn’t all that rigid and I have to creep up on precision diameters due to flex in the system. I have shaved the last few 10ths off with spring passes and had a great finish. I do mostly aluminum, though, so that may be the difference. I use HSS tools exclusively. Just throwing that out there FWIW.
Tom