Asian names (Korean in particular) are very hard to look for, considering the number of the way the name could be written in Latin alphabet.
When you adding a cast, you see a lot of options that could be (or not) the actor you look for. For example https://i.imgur.com/zpgMkdu.png
However, when you trying to do the site search, to look who are those suggested actors, you get zilch https://www.tvmaze.com/search?q=Park+Ju+Hyun.
Kind of weird, one search gives you a lot of options, and the other gives you not a single one.