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I've heard this from others and I do notice it at the edge in most ultrawidefields, but I find it's related to the position of the eye and head in relation to the exit pupil. I think most of the CA can be corrected with eye placement out to fairly near the edge. If you hold your eye in the wrong spot, it can even be seen fairly close to the center.
It may be (I'm speculating here) related to that characteristic called "spherical aberration of the exit pupil" which, though controlled in most ultrawides, is not totally eliminated.
Whatever the case, since it can be tempered by eye placement, it is not usually considered a big deal.
But if you let the planet drift to the edge, a narrower field eyepiece may be better.
But which one?
Aye, there's the rub.
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