Diagnosis
The 6GHz band is highly susceptible to physical obstruction and even minor interference. A single bad placement or new object can cause it to fall back.
How to Fix It
First, check the placement. The satellites need a clear, direct line-of-sight with the router, not through walls, TVs, or large furniture. Reposition them so they face each other directly. In the app, go to Attached Devices, select the satellite, and force it to use the 6GHz backhaul.
If That Didn't Work
A firmware bug can cause unstable 6GHz links. Check for a firmware update in the app. If updated and still problematic, try disabling the 'Fast Roaming' and 'Implicit Beamforming' options under Advanced Wireless Settings as a test.
Heads Up
6GHz has terrible penetration. If you absolutely need walls between units, expect it to use 5GHz. The performance drop is normal in that scenario.