I've isolated exactly when this occurs. If I use the preflight panel, then go into overprint preview mode, it happens when I exit overprint preview mode in the preflight panel. At that point, the top ruler becomes two rulers. It happens maybe 70% of the time. Furthermore, when this happens, the cursor becomes offset by the same size as the lower ruler. The cursor is off by something like 3/4 inch. It's easy to resolve it. Control/Y and Control/Y again and it resolves, but it keeps coming back. This only happens on my Mac which is from around 2014. It happens through the last few Illustrator version updates. It does not happen on any of my PCs.