@Kevin_Z I had the same issue earlier, see here: Archer C6 v2 Wi-Fi 5 GHz channel width issue - Home Network Community (tp-link.com) However this seems to work now properly with the latest beta...

Hi, my Archer C6 is set to 40 MHz channel width in the 5 GHz Wi-Fi settings to avoid interference with neighbor Wi-Fis. I have noticed that from time to time it starts to use 80 MHz, although the...