Teryaki sauce becomes viscous after boiled: does that mean the sauce is unhealthy?

I recently cooked chicken in a Teryaki sauce/marinade, and it turned out great. I cooked the chicken on both sides, and then cooked it in a bath of Teryaki for about 3 minutes. The Teryaki bubbled a lot.

I let the Teryaki sit there in the pan, after cooking, for about 24 hours, and it turned into a tar-like, viscous substance. It reminded me of a video of a guy doing the same with coca cola, which had the same type of affect, which brings me to the question: does this mean the Teryaki sauce is unhealthy, or is the water just vaporized, making the remaining substance a very viscous fluid? Health is the concern here.

Also, is it bad practice to cook it in a bath of Teryaki like this? Would marinading it be a better alternative? How long should I marinade it for?

This is Yoshinoya's Sweet & Sour (Teryaki-like) sauce, from Costco. Very tasty.

Thanks!