Did you remove the vent tab before filling carts? With vent holes open, some ink can flow out of the air vent channel/hole, but I've only experienced a few small ink bubbles, not a puddle like you experienced. As long as ink isn't draining from the exit valve when the cart is removed from the printer, and the thin side membrane isn't damaged, then the capsule should be fine to use (but, please do let me know if you continue having any problems or strange experiences with it).
Yes, Don's storage recommendations are good, and essentially what we do here (plug vent holes + store carts upright in a sealed zip lock bag).
Best regards~ Dana