I’m getting it too – and for me it’s related to the which text editor I’m using. I have two themes I’ve worked on that are throwing that message. One uses a proprietary visual composer and the other uses WPBakery Visual Composer for pages/posts.
If I am in the WP’s Classic Editor interface in the VISUAL tab, it gives me the notice about navigating away from the page when I publish or update.
If I’m in Classic Editor | TEXT tab, I don’t get the error.
If I’m using the theme’s Visual Composer I get that error when I publish/update, but not if I use the Composer’s save button.
I just now thought to try a different browser, so I haven’t yet – but I’m using Chrome.
Hope that helps figure this out – quite annoying! ??