Pages 14.1 won't move an image back behind text
I'm using Pages 14.1 on macOS 13.7.1. I've got two documents open. In one, I've got a title page image that is behind the text on the title page. In the "Arrange" inspector, that image is configured as "Object Placement: Move with Text" and "Text Wrap: None". The "Back" and "Front" buttons in the inspector are usable; the image has been moved all the way back, but I can click "Front" and then "Back" to move the image up and down relative to the text. So that's great. In the other document, I have the same configuration: a title page image that I just inserted, "Move with Text", and "None". For this image, in this other document, however, the "Back" and "Front" buttons in the inspector are dimmed and unusable. The image is in front of the text, and that apparently cannot be changed. I've quit and restarted the app, I've deleted and re-inserted the image; it remains the same. The "Send to Back" menu item is also dimmed when the image is selected. Both images are PDFs.
If I insert the same PDF image used in the first document into the second document instead, I also cannot move it to the back in that document. Similarly, if I go to the image in the first document and click "Replace" in the image inspector, I can replace it with the title page image for the second document, and still move it back and front. So the problem seems to be with the document somehow, not with the image; the first document allows this movement, the second document doesn't. Anybody have any ideas on how to fix the second document to allow me to do "Back"?
MacBook Pro 16″, macOS 13.7