@MrYM

Band Steering wont affect this, unrelated technology so you can re-enable this ok.   Fast Roaming is just roaming, it could be a roam causing it but as you have ruled that you its unlikely.

My question is, does this cause you an issue?  If not perhaps best leave it..  let me explain.

Each time you add a node (another Deco) that new node will only get 50% of the speed of the Main Deco (this is just how WiFi works).   To keep the maths simple lets say your main Deco is 1000mbps speed, the extra node will get around 500mbps.

So im Device 1 and i can see both nodes, the Main Deco is around 60% signal, the Node Deco 95% signal, connect to the Main is the best option as 600 out of 1000, is better than 450 out of 500

Although oversimplified, this is the kind of decision that the Deco will be making for you as it chooses to roam your devices.   If its working, personally i wouldnt get too hung up on the node it connected too, should performance change it may well move it again.     Fast Roaming, best keep it enabled as roaming will still happen with this OFF, but will be slower (its in the name really). 

  @Philbert Makes sense. Thank you!