You can submit a bug report with a sample project, but I think the short answer is that the message is harmless.
Keep in mind that macOS has to go to a lot of trouble to display a save/open panel in a different process (the "powerbox"), but make it look like it's part of your app. (For example, delegate methods have to run in your address space, and any accessory view, which may have custom event handling in your process, has to be — or appear to be — part of the window that's in the powerbox. It's a very fragile mechanism.)
I suspect the message arises because of some initial assumption about the view layering is found to be incorrect, and there's an exchange of information between your process and the powerbox to fix things up. Vibrancy is itself pretty complex.