@Dean_B 

Hi, nice to see you again.

I came across a similar discussion earlier, and there is one answer that I found quite useful,

https://www.googlenestcommunity.com/t5/Chromecast/Chromecast-4-0-can-t-connect-to-wifi-WPA2-WPA3-mixed/m-p/91448

Mixed Mode means both WPA2 and WPA3 capabilities are advertised out of the Access Point.  It means that both WPA2 and WPA3 clients are able to connect to the SSID.  Those clients which support WPA3 will use it, those which do not support WPA3 can still pick up on and connect using legacy WPA2.

Therefore it isn't relevant if the smart devices supports WPA3 or not.  In Mixed Mode,It should be able to connect using WPA2 as it always has.  A well-behaved wireless client will ignore capabilities and beacons that it does not understand.

Like in your case, the Sonoff devices should be ignoring WPA3 but apparently, they don't.

So I don't think this issue lies on Deco X20-4G.

Thank you very much and best regards.