It doesn't sound very clear to me, either. I don't think the TTS is pronouncing it incorrectly, but it sounds like 어두워 and 요 were recorded separately. 어두워요 is, as you know, literally pronounced [어두워요].
I hear yo. If you can test it again to see if it's the same, either they've updated, or there is some weird audical illusion. Lol if that's even a thing, though i did read that what you hear can be altered through time even though the sound remains exactly the same.