Reproduced on LV2013. See attached VI. Changing the Y-scale of the plot during run-time should reproduce the problem. It seems that the cursor's y-scale is not updated until you move the cursor manually.
A workaround is to catch the plot attribute change event and update the cursor list manually. I've implemented this in the attached VI. Enable the Fixit? button and then change the plot's Y-Scale and the cursor should snap to the correct scale.
The attached VI is saved back to version 2011. Let me know if it works for you.
Ok... I can't attach files... Here it is in dropbox: https://dl.dropboxusercontent.com/u/8640811/XY%20Graph%20Bug/XY%20Graph%20Cursor%20Test.vi