A solution to envelope printing with iPad and Pages (and other apps) - page size mismatch
A common problem has been printing envelopes with iPad ( NOT just Pages). The output to the printer sends a full-sized sheet of paper, not the envelope size. The solution may be at the printer. Check to see if there is a page size setup at the printer end. Mine only showed up when I loaded in envelopes, and it was hard to see. After making the adjustment, the envelopes printed perfectly.
iPad, iPadOS 14