I was having the exact same issue on my Mac and finally found an easy, semi-hidden solution.
In the mailings tab, I started in Envelopes
I left printing options set to "Use settings from your printer" (after spending an hour fumbling with the feed settings in the custom settings option
In page setup, I used Envelope #10for paper size, with Landscape orientation

Click Print...
Set all print settings as you normally would for your envelope size (#10 for me)




The solution for me was hidden in the
paper handlingmenu and involved selecting scale to fit paper size, then changing the
destination paper size to Envelope #10

It seems highly redundant that you need to select this after selecting the paper size in page setup, but at least it solves the problem (in my case).
Finally save all these settings as a print preset to avoid future headaches...
Now it should print correctly on a center-fed envelope assuming you feed it in the correct orientation (might take trial and error). Screenshots of print window attached for reference.
Hope this solves the issue for other Mac users in this thread as well!