I have the exact same issue, getting this error every few seconds:
Aug 20 13:44:33 acsd: eth7: Selecting 5g band ACS policy
Aug 20 13:44:33 acsd: acs_init_run(1111): acs_start eth7 cannot select chanspec with the wrong info
It's entirely related to AiMesh. I have an AX86U as my main router and an XT8 as the node. When I remove the XT8, the error goes away. When I re-add it, it comes back. On the AX86U, I have selected both my 2.4GHz and 5GHz channels, so nothing is on Auto. I do have SmartConnect enabled though, I need it to stay on unfortunately. On the XT8 node, I can't control any channels. So my guess is the error refers to the XT8, not the main router, i.e. the XT8 is having trouble with its own auto channel select.
This just randomly started happening, it wasn't an issue before, maybe a firmware update did it. I've tried rebooting the entire AiMesh, and removing and re-adding the node, neither helped. I haven't factory reset the main router, but I really don't want to redo all of my settings again.
It doesn't seem to have any effect on performance, everything still works fine, other than the usual bugs of DFS channel selection that I won't go into here. The only thing that seems to have changed is that 160MHz doesn't work anymore. If I set 160 MHz or 20/40/80/160 on Channel 100 (the node doesn't support the lower ones), it just doesn't work, it immediately makes Channel 124 unavailable for 30 minutes and that's that, and again I'm pretty sure that's related to the XT8, probably the same bug that's causing the repeating error in system log. I hope Asus fixes it.
I think with AiMesh, pairing things that weren't designed to be paired together, like in my case a dual-band AX86U main with a tri-band XT8 node, will often result in bugs. Asus QC is definitely not great.
Edit: Weirdly what fixed it was when I set the 5 GHz channel in the main AX86U router to Auto. It automatically selects Channel 100 which is what I want anyway, so it's fine for now. But it still won't do 160 MHz, even if I force 160, it just does 80. I don't know why, it used to do 160, but it doesn't want to anymore.