I had the same problem with my XT8 mesh... week after week the backhaul will switch over to 2.4Ghz and stick there. I tried setting Roaming on the 5Ghz-2 off, but no use. Then I gave it some more thought ... and set Roaming back ON, and also setting 'Disconnect clients with RSSI lower than' -90 dBm. Did the trick for me. Wifi Analyzer app always showed the RSSI location at my extender node at -65dBm to -70dBm which is quite close to the default threshold setting of -80dBm, so I figured that if I set the threshold RSSI to some value which it'll never cross normally, only glitches will throw the backhaul back to 2.4Ghz. With the roaming on, the Mesh will periodically check for 5Ghz-2 signals and reconnect to it. Not sure if this is exactly how the roaming on Asus AIMesh works, but this setting certainly worked for me.
Over the last 2 weeks, I had encountered a couple of times when internet slowed, so I quickly logged in to the web admin interface to check. True enough, the the backhaul went to to 2.4Ghz, but after 2 or 3 minutes, it goes back to using 5Ghz-2 again. My 2 nodes are separated by a concrete wall and they are 12M away from each other. Give it a try and see if it works