What is your OS?
Try to update your graphic driver.
I messed around with the images that were a problem and quickly realized they were not a standard aspect ratio. I switched templates and the problem disappeared
However, I can not reproduce it, no matter what template I try .
It's a Lightroom feature, not a flaw.
I have experienced the exact same issue and can provide some more detail. For me this always and consistently occurs
when using the template "Crop to Fit" when the collection contains images that have had Develop Adjustments made in Lr Mobile (on iPad). The problem images are always edited in Lr Mobile however not all images edited in Lr Mobile are a problem, for example I do not have problems with images that have only been cropped in Lr Mobile.
One workaround is to simply choose any other Slideshow template.
Thanks. Switching the template solved the issue. Adobe should fix this.