SDK: LrDialogs.presentFloatingDialog() remembers position but not on which screen?

I have added a second screen mode to my Focus Points plugin, enabling it to run on a second monitor while the user works in Lightroom. In this mode, the plugin runs in a floating dialog box that instantly responds to changes in the current image.

This works beautifully on Windows. The floating dialog remembers its screen position and size, when switching images and across Lightroom sessions.

On macOS, the floating dialog remembers its position and size, but not the screen to which the user has dragged it. The plugin window always appears on the same screen as Lightroom. If you drag the window to the top left on the second screen it will appear at the top left on the screen where Lightroom runs. 

Does anyone know how to make this work?