If there is white text on a colored background InDesign thinks there's nothing at all
If there's only white text on a page but this text has a colored "paragraph shading" ("Absatzschattierung" in German), InDesign does not recognize this color and thinks the page is still empty.
I reported this in the old "wish form" about four years ago: https://rainerklute.de/bug-report/
The dialogue shows a way to solve this but why is it not implemented?
Rainer Klute commented
Mac OSX 10.14.6