Most Shih Tzus I have seen up close always appear to be wet and runny in the face. Maybe it’s just the color and texture of that muzzle fur. Runny around the eyes too, for the adults. They remain popular, so, maybe that ‘Wet & Sneezy’ look doesn’t bother most owners.
I'm not sure where that comes from. We've owned four in the last thirty-plus years and never ever saw what you describe.
They were all smart, loving pets! Plus, even though I love my kids, Most Shih Tzus never move off, go to college, call home for more money, or wreck cars :-)