It's possible to select an object that isn't actually there. It's easy to reproduce.
Make sure you have some object on any page.
Scroll to a different page, but don't focus it yet by clicking on its canvas.
Select by dragging, making sure you intersect with some of the areas the objects occupy on the focused page.
You have now selected an object on a different canvas.
While this is not a major issue—it can be easily remedied by first focusing the other page—it often causes confusion (is there some invisible object here?) or annoying breaks in workflow (having to cancel your selection and reselect).
I'm on macOS 12.1 with Affinity Publisher 1.10.4.
affinity-publisher-selection-bug.mp4