Be sure to use very cold butter and milk. Freezing then grating the butter really helps. At the very least, keep it refrigerated until ready to use. Keep milk refrigerated as well.
Try not to overwork the dough. Stir just until no flour streaks remain.
Every air fryer is different. Check the biscuits for doneness at 9-10 minutes. If you feel they are getting too brown, drop the temperature to 320 F (160 C) and finish baking until a toothpick inserted in the center comes out clean.
Do not lay loose foil over the top of the biscuits as they are baking. An air fryer works by blowing hot air over and around the food. Loose foil will just blow around in the basket, and can even lead to a fire risk. To prevent too much browning, it is best to simply lower the temperature of the fryer.