[EldRick]

I had one that showed the dreaded grey bar, and called Kobo for a return. The CS rep had me do a factory reset on the Forma, and the artifact was reduced by 80%.
While it was dark enough to be distracting before, after the full restart it is acceptable - slightly there if you look for it, but not enough to be annoying.
I conclude that this may be a calibration issue - has anyone else tried a factory reset?
Here's what I got from CS
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Performing a factory reset
You can perform a factory reset to restore your eReader to
its original settings. After a factory reset, you’ll have to set
up and sync your eReader again.
If you perform a factory reset on your eReader, you'll:
• Remove all books from your eReader. Any books you
purchased from Kobo can be re-downloaded from
the Kobo Cloud after a factory reset.
• Lose any bookmarks and annotations you've made in
your books since you last synced your eReader.
• Sign out of your account on your eReader.
• Reset all of your settings and preferences.
• Return your eReader to its original settings.
1. Go to your Home screen.
2. Tap Home at the top of the screen.
3. Tap Settings.
4. Tap Device information.
5. Tap Factory reset under Advanced.
6. Tap Reset Now

Performing a manual factory reset
If a factory reset didn't solve your problem, you can try a
manual factory reset to restore your eReader to its original
settings. After a manual factory reset, you'll need to set up
and sync your eReader again. Before you start, charge your
eReader for at least 1 hour.
1. Press and hold the power button until you see a
black 'Powered off' screen. Note: If you don't see
the 'Powered off' screen, go to the next step.
2. Press and hold one of the page turn buttons.
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3. While holding the page turn button, press and hold
the power button.
4. Wait until the screen flashes and starts the reset
process. It usually takes about 30-seconds for the
eReader to start the reset process.