Hi pink kitty
Actually I'm also not sure whether is it coincidence or because of the milk powder, dear son also facing watery stool after I changed him to Friso 2 when he was 6 months' old. Each time during the milk feeding, he sure will pass out his watery stool. I've noticed this happen during milk feeding time cos when I feed him his normal solid food, he's ok. After which, I actually gave him soy based milk as advised by me pediatrician and only just recently I switch him back to cow's milk.
Perhaps, you try to hold back on your apple puree? Try giving her abit of banana and see how. If you r uncomfortable in not feeding her any solid, maybe just let her have some plain porridge cos at this stage her main nutrients come mostly from the milk.