Orthodontist cut Invisalign tray to correct open bite--said molars would inevitably come together.

After completing invisalign, my back molars weren't touching as my canines and front teeth hit into each other when biting down. My ortho did a set of 6 more trays for the top and bottom. After completing those, the bite improved, but molars still not touching. The ortho then cut the last tray I was wearing so it only covered my front four teeth and canines and said the molars would come together. He told me to wear these for 6 weeks. It this normal? Should I have gotten more trays instead?