@AlonIsrael There have been numerous discussions here in this regards. It always comes down to the signal path, signal strength and interference in the Wi-Fi frequency. 

Fo example I have a six Deco M5 Mesh. The Main Deco is connected to a 24port Switch. I have over 18 wired connections and over 35 Wi-Fi connections.

Example:

An M5 in my detached garage, which is supposed to connect to the Primary Deco router, through a TP-Link AV1000 PowerLine connection. 

Most often when I check the backhaul connection, it does show Ethernet, which means that it is using the PowerLine connection for backhaul. Occasionally the PowerLine connection needs to be reset and when that connection becomes slower that a Wi-Fi connection, the Garage Deco connects to a second floor bedroom Deco M5. My Main Deco and Switch are in a lower level through many concrete and stone walls. 

If I reset the devices on both ends of the PowerLine connection the Garage Deco will eventually switch it's backhaul back to the faster "Ethernet" connection.

Bottom line is that the Deco backhaul, along with client connectivity, will select the "CLEANEST Signal", rather than the obvious visually closest connection.  There are so many devices that emit radio interference, which we are not aware of, that will cause one connection to be chosen over another. 

Trust the software engineers who wrote the Deco Mesh firmware to ensure that the connection chosen is the best option. 

  @DAServices 

FWIW, my experience is that the logic for choosing the signal source in M5 was/is entirely flawed, and at least over WiFi - seemed to always connect to the main deco regardless of signal quality/strength.  I'd love to be able to trust the engineers to do the right job, but I can't ignore reality that it just doesn't work - and very evidently so.

Thankfully, the new M5 firmware allows manual selection of the signal source, allowing you to override the basically hardcoded always-main-deco signal selection.  In my parents' house - this transformed the 3rd deco from a useless piece of white plastic, into a fully working node running at full speed.

I can't wait for them to make this available for X20, and when they make it's finally available for the X60 - Alon's problem will be solved too.