This following article by Yonhap News tries to attribute the drop to problems with the ruling party. Bull.
Yoon's approval rating falls below 40 pct amid turmoil at ruling party
Next article from Korea Times is more objective.
Yoon's approval rating falls below 40% for 1st time: poll
In spite of ALL the extremely FAVORABLE publicity from almost ALL Korean MEDIA, Yoon's approval is dropping. Could go down to around 30%, with support from only hardcore conservatives.