It’s possible the Envelope Feed orientation/direction is causing the problem.
Using either the Mailings > Start Mail Merge > Envelopes or the Mailings > Envelopes, for me, doesn’t allow an object (Text Box in my example below with dashed boundary line and relevant wording) to be seen fully in the top right corner in the Print Preview and it appears there is a large margin ‘blocking’ the view and potentially the full ‘stamp’ on an envelope.
It could be the envelope setting feed needs changing and the envelope feed is physical changed to compensate.

I have created a short video to show you my results of creating an envelope and pasting in and positioning a previously created stamp guide.
The feed used initially is my default printer option which works fine ordinarily on sheets and even envelopes that just have the central main address and the return address in the top left corner.
Usually, nothing would be placed in the top right corner except the actual stamp.
Click the image below to see how changing the feed in the envelope settings changes the view in the print preview.
I must add that at the time of posting, I have no printer attached/available and can’t actually fully ‘test’ my theory so you will need to change your settings and how the envelope is fed in the tray (as an experiment) to see if this helps you out.
Note: Placing an object too close to the edge of a page/envelope will have the Printers own border (white – approx 3mm) appearing and even a no border option in a printer that supports this is highly unlikely to support an envelope size.



PS – I wouldn’t worry too much where a stamp is placed on an envelope. I appreciate it may not look good and arguably not very professional, but I have seen received post with stamps placed everywhere but the ‘expected’ place on an envelope and it still gets through. The post guys don’t like it, but they deal with it one way or another!