As noted in forum post https://forum.getkirby.com/t/previous-next-item-navigation-in-the-panel-default-behavior-isnt-real-intuitive/27330 :
I have a page with sub-pages using two different templates. Everything was working fine except that the panel next/previous navigation between the sub-pages only connected sub-pages of the same blueprint. (Essentially I had two sets of siblings, as far as next/prev was concerned.)
I did finally discover in the Page blueprint reference (https://getkirby.com/docs/reference/panel/blueprints/page) that I needed to specify broader behavior, which fixed my issue:
navigation:
I guess I’m a little surprised that the defaults are not “all.” I’d expect to need to look up a solution to lock things down in a specific way – not to open up what seemed like puzzling, restricted behavior.