Well yes it is-- Cale used the pure Korean words for "happy" (라온) and "dragon" (미르), which you may notice does in fact have 2 syllables. It is in fact Miru, though perhaps Mireu would be more precise.
I'm not sure why the translator decided to put their foot down so firmly.
Sure, it follows some translation "rules" for other similar things, but it is at its core a pure Korean word, so changing the pronunciation to suite English more seems to be in poor taste, in my opinion.