Are you using Brother drivers? If so, I suspect an incompatibility. Try to set up the printer as an AirPrint printer. Do that by deleting and reinstalling it in System Settings.
Brothers own driver probably has an option where you get instructions how to manually flip the papers. I doubt that comes up in the generic AirPrint driver.
But here is how to do a manual duplex, for all printers:
1) Print all even numbered pages (back sides) in reverse order (that way ”page 2” will be on top ready to get ”page 1” on its front side)
2) Reinsert printed papers in tray aligned and flipped correctly. You will have to trial and error on your model 🙂
3) Print all odd numbered pages in the natural order.
Starting with even pages first makes it easier when there is one extra odd page at the end that should be printed on only one side.
