Just a follow up to say that since I've disabled the 160Mhz band things have been really solid for me and my nodes stay connected using 5ghz-2 for backhaul.
It's difficult due to the layout of my house. The position of router B currently gives me the best coverage and I can't move router A.
Why would being too close cause problems? Why would that force it to drop back to 2.4ghz for backhaul?
I have two units.
A is connected to ISP router by ethernet (I've disabled WiFi on the ISP router).
B is connected to A via 5ghz backhaul with RSSI = -32dBm
I've been having exactly the same problem described here. After a day or so my second XT8 starts using the 2.4ghz channel for backhaul and my network slows down to a crawl. The 5ghz-2 dedicated backhaul has great signal between the two nodes.
I've just tried unticking the "Enable 160mhz" option...