Although this side effect can be resolved by the hands of Cyrus after a woman cultivates the technique past the mortal realm under him. But before that, one needs to be very careful.
As for why this accrued, it was by overdrawing the Yin essence. A mortal woman didn't have a large Yin reserve, and they needed to rest to recover them. Although the DST can make those reserves larger, Cyrus's old godly self never really carved the yin of a mortal before.
But the current Cyrus was different, and he was like a bottomless pit for LXP, so he was now worried that this side effect might occur in Rita if he drew so much Yin from her on a regular basis.
Furthermore, DST was designed such that the woman won't even know they were walking on a deadly path, and without Cyrus, they were doomed to fail.
Every creator creates their techniques this way to have complete control over them and also to protect their techniques from falling into the hands of others.
