Same! That’s what makes me think I like him best as a secondary or minor character. I really liked him in Nine where he played the younger version of the lead (and where I first saw him) and I absolutely adored him as Myungsoo in Heirs. But he didn’t really work me as the king in Hwarang and he’s not working for me here. As you say, he just feels kinda bland even though his character is hella quirky.
I’m not sure if it’s him or the character, since I could see a few actors making the character come alive for me. But I also know there are plenty of others that seem to like him in the role, so maybe it’s just me. *shrug*