I also felt like something similar happened to ChatGPT. A few days ago I asked it to rewrite some Korean text with Hanja, retried many times, but it kept spitting out the same text without changing a thing. After several frustrating attempts, it finally spat out something with Hanja, which according to my deduction with the help of Google Translate, was only partially correct. A few months ago though, it could come up with something that’s mostly correct. Sad.

PS: Before anyone replies to me in Korean, I should note that I don’t speak Korean at all. I just happened to have stumbled upon this Wikipedia article about Korean Mixed Script.