The deflation MIGHT have something to do with how old your baking powder is, too. I have a sneaking suspicion that the longer it sits in your cupboard the less potent it is. But I also have no facts to back that up - just a hunch.
Just tried this - they're in the oven now. I used lemon pudding and added a wee bit of poppy seeds (checked nutritional info first and I'm good!) to get a lemon poppy seed muffin cake type thing.
I didn't end up portioning them into nine but did four instead - will see how they turn out!