How do I make stir-fry sauces that don't get watery and thin as leftovers?
This is pure conjecture:
It's possible that the issue is in how the vegetables are cooked -- commercial restaurants have crazy hot burners so the vegetables are cooked on the outside while still firm in the middle.  It's very difficult to achieve this with a stove at home.
The closest that I've managed to get to it is by cooking in batches ... an ingredient or two at a time (or half of a given ingredient when I'm cooking for a crowd) in an oiled skilled that's heated to smoking ... and then move it to a bowl once it's cooked, and cook the next batch once the skillet is back to smoking hot again.  (I deglaze in between batches, with mirin or similar, if needed)
Another possible issue is the ingredients.  There are a few foods out there that will break down gels due to enzymes in them.  So if you're making sweet and sour dishes with fresh pineapple, you're actually better off using canned as they've been cooked thoroughly enough to deactivate the enzymes.  You should also only taste your sauce with a clean spoon (or using the two spoon method), so that enzymes from your saliva don't contaminate the sauce, as that can also break down gels given sufficient time.