Wait maybe I get the point, I think you can't do a Set Y-value of a cursor linked to a single plot. For some reason LabVIEW doesn't generate an error on such action (which it should IMHO, this is a BUG).
Yeah, that's what I couldn't get to work in my example. I would agree that it looks like a bug to me.
I filed a bug-post
I was able to reproduce this issue with a simple VI; this is reported and under investigation under CAR# 165613.
Thank you so much for bringing this issue to our attention.