As the title says: the SS tubing is not 100% round, so either one of the lower bearings does not make contact or, if I fasten it, it is rubbing against one brace where the tubing pushes it out by a small amount. Can I sand the brace down a bit? I will not be able to change the tubing (rotated it once already) and the bearings should make contact… 
First be sure that you aren’t over-tightening the adjustment bolts in the core. You want them to just make contact with the rail.
Then, if you are still having trouble with the core contacting the braces, you can sand the braces down a little, too. I’d avoid going through more than a single wall of the plastic if possible. Also try to avoid over-tightening the screws that go through the front brace, which will distort the positioning of the tubes relative to the beam and make the tube clips more prominent.
Those screws can exert tremendous force on printed plastic, far more than it is reasonable to expect it to take.
Thanks Dan, I tightened all of the screws by hand, and it’s only one brace, the others are fine. At least the bearing is now always touching the rail. Sanding it is. 