Have a pair of ET8s with this same issue. 6E from the router but not the node. ASUS finally released new firmware (3.0.0.4.388_23759-gfd1c030) for the ET8 yesterday which did NOT fix this unfortunately.
I am using AP Mode with ethernet backhaul (with the toggle set in AIMesh System Settings) and at first thought this issue was fixed with the new firmware. However I am still seeing the 6G signal disappear from the node a few minutes after restarting/rebooting it. 6G continues to work fine on the router.
I did notice (on this and the prior year-old 386 firmware) that the 6G BSSID of the node in the GUI is one digit off from the ethernet interface in the node that it appears to be configured for. Ssh to the node and look at ifconfig. The MAC of my node's eth6 interface ends in B8, but in the GUI the BSSID for 6GHz on the node ends in B9. In the router the eth6 MAC and the 6GHz BSSID are identical, each ending in A8 in my case.
This may have nothing to do with anything, but the MAC and 6G BSSID match in the router and they are one digit off in the node. If it is that simple I would think ASUS could easily fix and issue an update to solve this once and for all. As many have said, everything else works so I have not returned the units - and this new firmware is a long-overdue, welcome update. But the ET8 should behave like an ET8, with 6G available everywhere. If we didn't want that we could have saved some $$ and went with the XT8. ASUS needs to fix this.