I’m having this problem as well- on my website (which used to have no problems at all) I have 5 pages of various page lengths, and 4 of them have different amounts of white space under the footer, but not the longest page. After clicking back and forth to compare page lengths, it seems the 4 pages with the white space are all trying to add space to make them as long as that 5th, longer page.
This is a new problem for my website, and I notice it has occurred ever since the Wix update that launched the “Sections” feature into the editor. I added a couple Sections to my site after it launched. My guess is that using Sections instead of just Strips is what’s causing the bug in page length.