I use my mouse scroll wheel to zoom in/out which is far more convenient than using the numeric keypad. In any event, zooming is not the issue. It is panning.
When a slideshow is paused, left click and hold on the image and then drag in any direction - the slide will disappear as soon as you release the LMB. Next, zoom in on a slide (via scrolling or numeric keypad), pan the image (LMB+hold down) - then release the LMB - the slide does NOT disappear (while zoomed). But then click the next/previous button in the Player Bar - the image will show very briefly and then disappear. This happens for every subsequent next/previous click. It also does not matter if you invoke the next/previous slide via the Player Bar transport buttons, or via the keyboard arrow keys - the slide will briefly show and then disappear.
It only occurs when slideshows are paused, and when you are using Standard View, Cover Art View, and Detached Display View (which are the views I use - I don't like/use Theater View). In display view, it does behave (mostly) correctly (slide does not disappear after clicking next/previous), but if you manually pan an un-zoomed view and then release the LMB, the slide will disappear.
if others could please follow the steps above and confirm that this is indeed what is happening, I would most appreciate it. It is 100% repeatable on my end.