I can’t say that it applies to all garlic/onion in the dry form but I’ve used a lot of different forms (powder, flakes and salt) as well as the same from different manufacturers and in all cases it caused the tomato pectin to gel. The best thing to do is to simply put it into a microwave safe container, like a small bowl, add some water or oil to the onion/garlic and heat it to 200F. You will need to use a thermometer if you use oil but if you use water this will be the temperature where you just begin to see some bubbles beginning to form prior to boiling. Then just add the heated suspension to the sauce and you’re good to go.
Tom Lehmann/The Dough Doctor