As fellow users, we can only recommend workaround solutions to the application problem. Any fixes that would make you happy need to come from the macOS Product team that does not participate in these public community discussions.
In High Sierra 10.13.6, the navigation panel appears across the bottom of the slideshow and eventually vanishes without input device interruption while the slideshow continues to advance at a quite rapid pace for practical reading consumption.
It would surprise me that the same engineer that worked on Preview in the High Sierra timeframe is still supporting the application, and that introduces other hands strangling the cat. Product management decisions that resulted in changes to the application may also have caused collateral damage to functionality, and that would require an attentive QA team in place to notice the aberration with slideshows.