Bottom layer not showing up in AI file placed into another AI file... so weird.

Hello smart people. I am working on a presentation, in Ai. In my workflow, all the creative development is done in external Ai files (so I can work at size), and then I place the development files into the presentation document, which eventually gets saved as a PDF for sharing with the corporate types.

In this case I'm working on corporate templates. The art (logos and type) is on a layer, and below it is a layer with white boxes on the artboards so the placed art will be opaque and I can put a drop shadow on it. 

Except... it's not. Opaque. The white layer isn't showing up. Other pages from the same working file are placed and are working as expected.

Tech specs: Ai 27.6.1, MBP (Intel), 32GB RAM, OS Ventura 13.4.1. Quit and restarted Ai, but not the machine yet. I did a full restart this morning when firing everything up for the day. Will do another restart after I post this. It's lunchime anyway. 

FYI, I can't share screen shots due to confidentiality, sorry.

Please advise, thank you!