Stuff like this has happened in the past, but it's usually because people have their second monitor in a different device. It's unusual if it happens when both monitors are running off of the same video card. I'd probably just recommend disabling preview.
RIght click the preview in OBS, and then select the option to disable it, and it may fix the problem, though of course it won't display the preview, but it's probably the best suggestion I have at the moment.