A family of Microsoft word processing software products for creating web, email, and print documents.
It seems as though this specific issue was only recently introduced in the latest update to Office. One of my clients started having this exact issue only last week after their Office apps were updated to the current version - Microsoft 365 Office Apps for Business Version 2202 Build 16.0.14931.20128. Prior to this update they could click on Mailings > Envelopes > enter the delivery and/or return address > click Print and it would only print a single envelope, which is the way it had been behaving for years. Since the update, now they go through this same exact procedure and it prints out a blank page after the envelope every time, no matter which printer they are printing to, and even printers that don't have an option to include a separate page. I was able to replicate this issue on my own computer with the same version of Office by printing the envelope to Microsoft Print to PDF and saving the document, then when opening the PDF document it contains the envelope page and the additional blank page.
As a workaround for the client I created an envelope template for them to use, but this is absolutely an issue with the Mailings > Envelopes > Print functionality.