Hello Woebgon,
Apple Support Communities is a fantastic place for help with your Keynote file and how it appears in iMovie. We're glad to know the file appears correctly when opening the file on your iMac with macOS Big Sur installed. To confirm, if you were to open the file in Keynote on your Mac Studio, does the file appear correctly? If so, have you verified you're running the latest macOS version? If not, you can update macOS by clicking the Apple logo towards the top-left, followed by System Preferences > Software Update > Upgrade Now. These steps can also be found in the following article.
Update macOS on Mac
To update applications, you'll want to open the App Store, click Updates, followed by Update next to the application. You can also find this information in the "How to manually update apps on your Mac" section of the following article.
How to manually update apps on your Apple device
Let us know how that goes!