Alright I can confirm that the keyboard layout depend on whether the Siri remote is "live" or not. But it's tricky to test as even if you are using another remote or controller, your Siri remote remain live for a good while.
I first tested with the Search page in Netflix, Hulu, etc with the Siri remote, and they all had the bar layout. I rebooted my ATV4 and only using my Logitech Harmony it still displayed the keyboard with the bar layout.
I suspected the remote was still linked with the ATV4 despite the reboot, so I put it in a drawer to make sure I don't move it at all and waited for a few hours. Then only using my MFi controller, I went back to check on Netflix, Hulu, etc and they all had the grid layout this time.
So because it's hard to tell if the Siri remote is live or asleep makes the keyboard layout a bit unpredictable, but there you go.