Hey Chris,
Thanks for reaching out in Apple Support Communities. We understand that when you combine pages in Preview on your Mac, the pages aren't displaying properly when viewing the PDF and you're seeing an error. We know it's important to be able to use this feature; we'd like to help.
Try starting your Mac in safe mode and test for the behavior there: How to use safe mode on your Mac
After testing in safe mode, restart the Mac normally and test for the behavior again. Should the issue continue, test for the behavior in a new user account next: How to test an issue in another user account on your Mac
Let us know what you find and please provide these additional details:
What is the error message you see?
Are you referring to these steps for combining PDF pages in Preview? Combine PDFs in Preview on Mac
Does this occur with all pages you combine in Preview?
Best Regards.