That is a good question as I see the same thing. Not sure if different brands of envelopes make any difference. For what it is worth it is probably the fuser causing this. It hasn't been that bad or I would have tried different fuser settings, but that might be worth a look.
Tech did everything possible. changed out pressure roller, new fuser belt, but no change in wrinkling.
Thinking it is 2 things. pressure of the fusing which is not adjustable, and env stock.
Tech is reaching out to others to maybe find a solution.