On Mac resizing the window on my second monitor moves the window to my primary monitor.

I found a solution to this issue. When modifying your preset do not select the option for "always center," instead use "select current location" and use 0 x 0 for width and height. In this way you will still have the screen be centered on the screen you desire without it jumping back to your "primary" screen. (Note: Adjust these setting on the screen/monitor that you wish for the dimension to apply, so that current location function centers to that particular screen.) For multiple monitor setups you can always get your window centered and filled to each screen using this method.

I'm confused. It works correctly in the current version but not in the beta.

I found a solution to this issue. When modifying your preset do not select the option for "always center," instead use "select current location" and use 0 x 0 for width and height. In this way you will still have the screen be centered on the screen you desire without it jumping back to your "primary" screen. (Note: Adjust these setting on the screen/monitor that you wish for the dimension to apply, so that current location function centers to that particular screen.) For multiple monitor setups you can always get your window centered and filled to each screen using this method.