I just measured how far my couch is from the dock and it's exactly 8 feet. Which begs me to believe 8ft is the sweet spot. Because the closer I get the less wonky the controls get.
I did numerous tests just now with different configurations, docked, tabletop, 6 feet, 10 feet, covering the joycon, uncovering it. What I found is hat obviously the further you get the less responsive they become. However the only really weird outlier in my testing was simply sitting 6 feet away, lounging with my hands to my side, neither joy con covered, and yet still after some time the sensing would just get completely wonky. Sitting about 4 feet away I noticed that if my hands are by my side but pointing at the screen everything's perfect but as soon as I drop my hands to a more comfortable position, the calibration once again gets weird and has a few dead spots. If I put the joycon behind my back for example, then the party's over. There's barely ever any response or all the inputs are delayed. That one is more understandable I guess since it correlates to the gamexplain video, but doesn't at all explain why it's still messing up 6 feet away when simply lounging.
I also opted to try some of these variables with the grip since I figured the grip would force me to point the joycon more so at the tv/dock but surprisingly I found out that it too had some issues. I tried holding the controller in many different ways, all comfortable ways like I imagined I would hold it if I'm playing for a long period of time. Simply holding the grip in front of me seems to work just fine, however if I decide to bring the grip up to my chest (for example when I'm laying down) so that the buttons are facing it and that the top of the controller is facing my face, it completely stops working. Right joycon can still rotate the camera but link simply won't move. So the position of the joycon seems to really play a large part in how it's detected.
Also, man I can't believe how long I just spent rotating a damn dot around a circle just to see if a controller is working properly! On a new console no less! I really love the joy con too and I'd rather use them instead of a pro controller so I'm hoping this signal issue can be fixed with an update.