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 plus a little bit extra time, and reduce less aggressively next time. It becomes more severe when you reduce too quickly[1] or too much or too often.
Even though it's functionally the same as real astigmatism (although rapidly changing on axis and amount), 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 ("if you don't notice it, or if you hardly ever notice it, or if it's really just something that the optometrist shows and you see a tiny little bit better"), it's also OK to leave it.
If you somehow created a very large amount of transient astigmatism, such as this person who had up to 4.25 D of myopic defocus, you might consider adding some cylinder correction, but only as a last resort.
- ↑ "So people that push the reductions get this transient astigmatism symptom that’s kind of a pain." https://endmyopia.org/ruis-glasses-and-headaches-9-75-to-6-00-progress/