At this stage I have only experienced the channel change on reboot as these aren't yet in production. I cannot get them to hold a custom assigned channel after a reload. With DCA enabled they are still assigned a channel that isn't in the DCA channel...

I am seeing this behavior as well (17.9.6, 9124AXE's, Mesh RAP/MAP). We need to custom set UNII-3 5ghz mesh backhaul channels and they shift channels randomly on reboot to non UNII-3 channels that are also disabled in DCA. Was TAC able to resolve you...