I don't think this is true. Archer C6(Qualcomm) works just fine with RE205(Mediatek) [OneMesh]. I think RE505X is designed to work with AX20, and RE605/607x is for AX50. I'm waiting for WPA3 for...

I disabled the 2.4 radio since I don't use it and the temperatures are way better.

With FW 1.0.5 beta by@Carl, I don't have this issue. It's been two weeks since I downgraded to this fw and the channel did not change.

I also noticed this issue, but it also happens with non DFS channels. If I select channel 48 the router reverts to channel 36. I am alone on the 5GHz band acording to inSSIDer. Archer AX50 (EU).