I have 3 Deco X20 units, basic network configuration.  Main Deco with 2 satellite units connected by wifi.  I set them up and placed them in their current location before adding any client devices.  I have 25 client devices connected by wifi to the system; cell phones, PC's, and various other smart home devices.  The client devices were set up closest to the Deco unit I wanted them to connect to.  Common sense approach. There seems to be no logic in how the various client devices get connected to the 3 various Deco X20's.  For example, an Amazon smart plug which is within a few feet of my living room Deco connected to the main Deco further away.  A google home mini, which is a little further from the living room Deco did connect to the living room Deco, which is fine.  I went into the client device settings for the Amazon smart plug and changed from automatic to preferred connection to the Living Room Deco.  Even after unplugging the Amazon plug and restartting it still connects to the main Deco which is further away.  That's just one example, I have 7 other devices that did not connect to the nearest Deco.  Instead they connected to the main Deco, even after changing from automatic to preferred connection. The only devices that seem to connect to the nearest Deco are our 2 cell phones.  And that may be because the cell phones are the only devices that have random Mac addresses.

I'm not even sure that the Deco app is showing the correct information.  For example, my laptop PC always connects to the family room Deco, regardless of proximity to it when I turn the PC on.  Just to see what happens, I went into the PC device settings and disabled the wifi LAN adapter resulting in no internet connection.  The Deco app still showed that the PC is online and connected to the family room Deco.  I had assumed it would drop down to the offline clients.  It does this even after force stopping the Deco app and cleaing the cache.

Further, I disabled the wifi Lan adapter on the laptop.  Connected the laptop by ethernet to the living room Deco X20.  Forced stop the Deco app, cleared the cache.  Opened the Deco app and it still shows the laptop connected by wifi to the family room Deco X20, it can't be!  I restarted the cell phone and the Deco app still shows the laptop connected wifi to the family room Deco.  Obviously with the laptop wifi Lan adapter disabled it is not connected by wifi to any Deco.  I then disconnected the ethernet to the Living Room Deco and enabled the wifi Lan adapter within a few inches of the Living Room Deco.  The Deco app still shows this laptop is connected to the family room Deco over 20 feet away.

Finally, I just disconnected power from the family room Deco.  The Deco app network map shows the family room Deco as disconnected.  But, it shows the laptop connected by wifi to the family room Deco.  How can that be if the network map shows that Deco is off line?

It appears that client devices with static MAC addresses will always show what Deco they were originally connected to regardless of changes made through the app or actual hardware connections.