Your use of lighting here is just phenomenal! I adore the sunset and I love the gentle touches of it upon Marcus too!
Yeah as someone kind of pointed out already, it could be anything about the face and what you feel personally might be off. I think if you moved the mouth and nose up a bit more, it might work and maybe made the mouth wider, but I am not sure. Honestly, I think because you change styles here and there (depending your preference) it can be hard to feel out what feels right for the style you're using.
It doesn't mean it's weird but it just means it might feel different in your case as you're not like the lot of us and switch from realism to semi-realism to an anime like style. Which is never bad because again, it gives your gallery such variety in terms of what you choose.