That's so weird! Channels 120 to 128 are legal in the US. It's as if they gave you the nerfed Canadian versions and depriving you of the 160MHz band in channels 100 to 128.
I live less than 6 linear miles from DTW, one of the largest airports in the US. All of the channels are open to me and my details have reported the same as OP (et al) since I set up AX88 a over a week ago. I've not yet caught it temporarily disabling a channel in the details or the main log. I have been leaving it alone mostly, I'll keep an eye on the "radar channel cleared" and see if I can catch it happening.. no reason to believe it is but don't know.
This thread is very helpful.
Channel Information
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Channel 36 A Band
Channel 40 A Band
Channel 44 A Band
Channel 48 A Band
Channel 52 A Band, RADAR Sensitive
Channel 56 A Band, RADAR Sensitive
Channel 60 A Band, RADAR Sensitive
Channel 64 A Band, RADAR Sensitive
Channel 100 A Band, RADAR Sensitive, Passive
Channel 104 A Band, RADAR Sensitive, Passive
Channel 108 A Band, RADAR Sensitive, Passive
Channel 112 A Band, RADAR Sensitive, Passive
Channel 116 A Band, RADAR Sensitive, Passive
Channel 120 A Band, RADAR Sensitive, Passive
Channel 124 A Band, RADAR Sensitive, Passive
Channel 128 A Band, RADAR Sensitive, Passive
Channel 132 A Band, RADAR Sensitive, Passive
Channel 136 A Band, RADAR Sensitive, Passive
Channel 140 A Band, RADAR Sensitive, Passive
Channel 144 A Band, RADAR Sensitive, Passive
Channel 149 A Band
Channel 153 A Band
Channel 157 A Band
Channel 161 A Band
Channel 165 A Band
I just verified, I bought the router from Newegg on 4/9/2020. Not sure if that plays into anything or not?
I've had mine just over a week (B&H), and do not have that option either. Its hard to know how old the unit on anyone's shelf it.
I think that chipset explanation is the most logical and Asus's KM is out of sync for this.