KSacres, how well did you mix the batter? I use a blender so it's pretty much pulverized together. Based on what you described, I'm wondering whether stirring vs. pulverizing may affect the baked outcome...
I have a similar recipe that I used to make in the '70s(?). Haven't made it in a long time, but mine calls for baking it at a higher temp which may be what you need to "set" the custard. I think Halo is right about the mixing time and I think the reason for using the blender is that it would be hard to get the Bisquick to blend in without overmixing. If you don't have a blender, you might try mixing the Bisquick with part of the milk until it becomes smooth and then adding it to the rest of the ingredients. Anyway, here is mine:
IMPOSSIBLE PIE
1/2 cup Bisquick
2 cups milk
1/2 cup sugar
1 teaspoon vanilla
4 eggs
4 oz. coconut (optional)
3 tablespoons margarine or butter
Preheat oven to 400 degrees.
Put all ingredients in blender and mix. Pour into buttered 9-inch pie plate.
Bake 25-30 minutes or until custard is just set.
Edited to add: This recipe was on the Bisquick box.