That seems to be expected for wireless backhaul. 6GHz has worse coverage than 5GHz. So using 6GHz as backhaul (or mixed 5GHz/6GHz) for Asus ZenWiFi BT10 will be kind of worse than using 5GHz for dedicated wireless backhaul with ZenWiFi XT8 in terms of coverage. You have to reduce the distance between the two nodes.
You may take a look at the following article. The following write-up is about WiFi 6E, but it should apply to triband WiFi 7 AImesh combo as well. I tend to think 6GHz or 5GHz/6GHz backhaul will still be useful in Singapore as we are mostly live in a small space. But we do need to remember that 6GHz band has a shorter range compared to 5GHz.
https://dongknows.com/tips-and-rules-on-picking-the-best-asus-aimesh-combo/
AiMesh wireless combo: Don’t count on the 6GHz band
It’s important to note that all Wi-Fi 6E routers are at least Tri-band, but they have no extra band that can be dedicated for backhauling—you need all three to host clients.
Additionally, the 6GHz band has a short range and can’t work reliably over distance or in a home with walls. (This is completely true; all that marketing hype about the band’s range is false.)
That said, if you use Wi-Fi 6E AiMesh routers, treat them the same as dual-band Wi-Fi 6 (or 5) hardware—use them via wired backhauling or expect reduced performance.
The takeaway is this: Don’t count on the 6GHz unless you live in a small or open space.