Hi Daniel,
Per the description shared, I understand you are not able to print the Word documents with Custom Print if the Word documents contain the Page Number and based on the build number provided, you must be using the latest version of Current Channel Office apps i.e., Version 2107 (Build 14228.20250).
If yes, I tested in the same version at my end and can print with a selection of Custom Print by using Page X of Y in the Header of the Word document. Since you mentioned you can repro the situation with a blank document, I suggest you perform the Online Repair to Office apps and check the behavior. Reference: Repair an Office application - Office Support (microsoft.com)
If the problem persists, can you please share the Product Information of your Word application and whether you can Print it as PDF with Custom Print selection?
Best Regards,
Chitrahaas