wi-fi agile multiband

I saw this setting on my new GT-AX11000, and read the Asus FAQ https://www.asus.com/us/support/FAQ/1042471/. I see that by default, it is disabled, when I turned it on and then added a non-AX access point (RT-AC68u) as an AiMesh node, it became disabled again. (The RT-AC68u does not have this...