Do Dumplings Mind Being in the Fridge?
It's not putting them in the fridge that is the problem, it is the fact they were cold when you put them in the stew. The effects of this would be:


Longer cooking time: the 20 minute time assumes that the dumplings are at room temperature, if they are cold they will take longer to cook as their internal temperature is lower. 20 minutes is a guideline anyway, you should test their doneness after 20 minutes even when starting from room temperature
Closer texture: cold dough into hot liquid is going to result in a closer texture because the outside is going to cook long before the inside gets warm enough to expand


The way to fix this is to take the dumplings out of the fridge and let them come up to room temperature before cooking, probably 30 minutes or so should do.