This won't be super helpful but I want to mention that I just kegged my beer for the first time, and the difference in head retention is astounding.
It was just a simple IPA with 10 lbs pale millet, 3 lbs biscuit rice, and 2 lbs buckwheat. That's the same amount of buckwheat I've been using for several batches, and none have had retention like this. And I'm not just talking about a lot of foam that can happen from a keg, but it actually sticks around longer than from the bottle. Maybe this is already well known but I don't understand the science behind it.
That being said, it could still be something else. I'll know for sure after I keg more batches.