81992 is when the current spate of auto updating started. I suspect if you had rolled back to the last firmware which was Aug 2020 you wouldn't have auto updated to 40451. I know I've rolled back to the Aug fw a couple of times the last week and have not seen it auto update at all; ac86u.I manually rolled back 40451 to 81992 on my AC86U and it auto-updated back to 40451. It's actually the first time I can remember my AC86U ever auto updating. Before installing 40451, I sat on 81992 and 82072 for quite a while without either updating. Something changed recently for me in terms of auto update.