The sample screenshot of a page in Deco app with Signal Source selection for Deco M5:

I noticed the following with Deco Signal Source selection, a feature that became available with firmware 1.6.0.

1. If Deco is connected by the Ethernet, Signal Source selection is disabled. 

2. If Deco is connected manually to Signal Source, and then connected by Ethernet cable - manual connection preference will be ignored. Deco will stay connected with Ethernet. Deco will remember its Signal Source connection configuration and will use it when no longer connected by Ethernet (i.e., when Ethernet cable is unplugged).

3. The WiFi sign after node name does not report actual signal strength to that node. It means, the list will include Deco nodes which are not reachable from Deco you configure.

4. If you forced Deco to connect to unreachable signal source, Deco will reboot itself and reconnect elsewhere.

5. I managed to trigger situation where Deco is connected to very weak signal source, and goes to non stop reboot loop. The way out of this situation was to temporary bring Deco closer to signal source, to force them establish good connection, make Deco appear online in Deco app. After which it is possible to fix signal source configuration for that Deco (such as turn Auto setting back on).

6. It sometimes takes few minutes for change in Signal Source configuration to propagate through Deco mesh and get noted in TP-Link cloud. For example, if you turned off Auto, quickly closed Deco app and opened again, you might see Auto setting still on. Just give it few minutes.

Got me confused initially, I thought there are bugs in Signal Source implementation, or configuration changes are not saved.

Finally, if you somehow messed Signal Source configuration for Deco node to the point it can't recover and LED stays red all the time, the way out of it is to connect that Deco by Ethernet cable to Main Deco or Satellite Deco. That will force Deco with misconfigured signal source to use Ethernet, and it should recover. After that, Remove that Deco from Deco mesh in Deco app, that will force Deco node factory reset and should remove signal source misconfiguration.