Archer BE550 networks disappear periodically

We bought two BE550 routers (V1, firmware 1.2.3), connected via EasyMesh (to use MLO backhaul). They have enough proximity for 6GHz backhaul.

But the 6e network will disappear for a few minutes, every 2 hours or so. This will happen near both routers, and drops at the same time.

The MLO and 5/2.4G networks stay up, but the 6e disappears. We use the 6e for phones due to battery drain on MLO.

Is this a known issue with the BE550?