Transient astigmatism

The official EM article says this effect occurs when you reduce too quickly and is temporary, so you should stop reducing and wait for it to disappear.

When cylinder overcorrection was first added to your lens-induced set of problems, your eyes remember when it was added in, as the visual cortex will compensate by negating that overcorrection past a certain distance. Once you've reduced to that point, you will temporarily experience that cylinder. (according to User:User)

Do not get cylinder correction for it, as you would just put extra load on your visual cortex for no reason, slowing down your progress.

If it's too noticeable, you can step up, but User:User believes that you must use lots of close-up near the edge of where the cylinder appears, because there would otherwise be no stimulus to clear it. If you have a huge cylinder only in the distance (very low myopia with huge cylinder), it's tricky, since stepping up to hide it would not solve the problem.