I confirm this is happening for me too using PM6 (3331) on Mac. For now I would recommend either closing with ESC or skipping the mouse-click to reset the zoom to 1x. I personally find it faster using Spacebar to open the Preview window, but YMMV.
If you double click with the image zoomed in, the Preview window will close and the next time you open an image with double click, it will open at the normal 1x. (I.e. The existing behavior could theoretically save a mouse click if you're thinking ahead. :-)
Mick