OK. I think I am getting more clues.
The gradient with the subtle tonality is actualy a 1920x1080 image that comes with an FCPX template.
It also happens to be a PNG file.
If I open it in Preview I see banding.
If I drag it into a new Motion 5 project I also see the banding. Until I press play and then stop again ..and then the banding goes away!
So i opened the PNG in photoshop. It sas It's 16 Bit. I see banding. I change the mode to 32 and I still see banding.
Then I change it back to 16 but I get a warning:
"Reducing the document depth can affect the appearance of layered HDR images. Merge layers before changing depth?"
I select Don't Merge. Now the document is 16 bit again...and banding. Then I change the mode to 8 bit. *no* banding! Change to 32 bit - no banding. And now no more banding no mater what combination.
I know it makes no sense, but it's true.
You can try it yourself - the file is called "BKG Gradient.png" and comes with FCPX / Motion 5
I think there's a whole lot more going on here than I had originally thought...
Love to hear thoughts on this.