Glad that you found a good solution, and Merlin firmware is indeed great, but the issues you were having were only reproducible by you from what I've seen of the many threads you've posted...
It really does sound like you had some extremely bad luck with your AX86U and a replacement or nuclear reset probably would've eradicated these issues.
It's been perfectly stable for me so far. Fingers crossed for Merlin support!
I did try a full factory reset, but that didn't solve any of the problems and really, I shouldn't have to do that anyway with a brand new router (IMHO).
Regarding the thousands of log entries like this:-
Aug 24 13:45:23 kernel: invalid dirty_p detected: ffffffc000000000 valid=[ffffffc03ce8e740 ffffffc03ce8e880]
Aug 24 13:45:23 kernel: invalid dirty_p detected: ffffffc000000000 valid=[ffffffc03c825840 ffffffc03c825980]
Aug 24 13:45:23 kernel: invalid dirty_p detected: ffffffc000000000 valid=[ffffffc03cae2140 ffffffc03cae2280]
Aug 24 13:45:23 kernel: invalid dirty_p detected: ffffffc000000000 valid=[ffffffc03cb34240 ffffffc03cb34380]
I received the following reply from ASUS:-
"The router system has multiply different accelerated mechenism. One of it is to mark the massive packets which had ever appeared, and then if the router recongnize the marked ones could let them pass more quickly. But the storage space is limited, so when the loading is bigger then ever, the old records would be replaced by the new records and when the router encountered the useless old records would show the messages like this. Basically it would not affect the transmission speed.
So in short, this is when it replaced old data with newly written data to internal memory"