I have the same issue with an Epson printer. Recently updated to Office 2016 from Office 2008.
I've had other issues since updating, all of them with a similar Microsoft non-answer and no help, telling me it is someone else's problem, like my Mac. Why is there no problem with any other vendor? Adobe, Google Docs, etc.??? Only Microsoft product.
Anyway, I've been pounding my head to figure out this envelope problem and there is none. The center feed option does exist in the NEW envelope setup but a) it does not work and b) it only is available for new envelopes not ones that you have saved.
This is factually a Microsoft problem that they are unwilling to fix and hoping people just move to Office 365 with a subscription. That's why I updated to Microsoft 2016 from 2008 because their auto update bungled my software and required I move to Office
365. I was able to get tech to help me remove it, but in the process it changed registry files on my Mac that permanently created a problem until I upgraded. Because of this and this envelope issue, I firmly believe that Microsoft is out to cannibalize its
older products in order to force people to move to subscription based where they make more money.