I have encountered the same issue on my flint and beryl devices. It doesn’t matter which firmware you’re on as it seems to affect all firmwares on all devices.
Make sure, in the repeater box click the cog and check the both/5g/2.4g option is on both or 5g. If so then:
The problem is the region selection in luci. Leave it on US because it seems to play fairer than other regions.
Once done turn off 5ghz signal in the gui and reboot the router. When rebooted login in and turn ON the 5ghz, select “modify” and check you have access to signals HIGHER than 48. If not then you probably need to reset the router to factory and start over.
Resetting is the short way around as its easier to reset and then program your wifi signals.