Transient astigmatism

Transient astigmatism is an effect that occurs when you reduce spherical and is temporary, so you should stop reducing and wait for it to disappear, and reduce less aggressively next time. It becomes more severe when you reduce too quickly.

Do not get cylinder correction for it, to the maximum extent possible, since you're just transitioning between two spherical values. If you do, 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 spherical, but if it's not that bad, it's also OK to leave it.

If you somehow created a very large amount of transient astigmatism, such as this person who used a high plus lens for distance, you might consider adding a small amount of cylinder correction, but only as a last resort.