Defaulting to the rear tray is a common issue across a lot of different printer models, and their solutions have pretty much all been the same, "adjust tray settings" or reset.
This does not work. I'm dealing with the same problem on the Maxify GX7020 and I've driven myself crazy messing with the feed settings, resetting the entire machine, setting the rear tray so that it's supposed to be set to envelope settings and it st...