It is not there that I could see either.
The only reason I can think of is because the door has a wider frame around it than the sidelight/window and then bottom is higher still.
However they can fit the standard horizontal/vertical grid in a door, so why not the editable one as well.
You can always create a custom door panel from slabs with the pattern you want, but it still won't be editable of course.
One of the forum moderators. Versions 6.5 to 25 Dell XPS- i7-6700 @ 3.4Ghz, 16GB ram, GeForce GTX 960 (2GB), Windows 10 Dell Precision 3510 - i7 6820HQ @ 2.70GHz, 16GB RAM, AMD FirePro W5130M, Windows 10
at least i'm not losing my mind looking for it and not being able to find it.
I had already resigned myself to having to create custom door panels, so in the end the door is looking correct, but could've been so much easier with the Editable Grid option, and of course there will be changes so will have to go back and amend the custom door panel at a later time.