I wrote off air fryer chicken thighs as rubbery for 2 years until my sister in law did one thing at her place in Reno
I had tried thighs maybe 6 times and got that chewy, gray result each time, so I told everyone the air fryer was only good for frozen stuff and veggies. Then at her house in Reno she pulled out a batch at 400 degrees for 18 minutes, no flipping, and let them sit 5 minutes under foil before cutting, and the skin was actually crisp while the meat stayed juicy. Is the rest time the real trick here, or is my old 375 for 12 minutes just way too short?
Did you ever check the internal temp or just go by time? I read on a cooking site that thighs really need to hit around 175 to 185 degrees before they stop being chewy, and your 12 minutes at 375 probably left them way under that. The rest under foil helps too, but I bet the longer cook time is what fixed it.