This problem is a fixed-pos box with a placeholder on the next page, which we misplace at 0,0. It has width:100vw; height:100vh so it covers everything on the page. Then we replicate fixed pos boxes on every page...

For comparison; it appears that Chrome duplicates the fixed-pos box only on pages that follows after the page where its placeholder is. Safari doesn't duplicate it at all (at least not in Print Preview).

This seems like a rare edge case though, and is arguably an author mistake, so I'm taking it off the priority list.

Fwiw, it should be easy to fix for the author, for example:

@media print {
  #side-panel { display:none }
}

or simply remove this empty element from the page unless it serves purpose...