Doing 1B for the first time and I jumped into the heart instead of dashing into it. This caused the heart acquired animation to play as normal, but instead of completing the level, I was moved back to the start of the room as if I had died or collected a cassette tape. Moving back to the final area, the heart was gone. Quitting to the menu showed the level never being cleared.. 

Fixed by replaying the end of the mission and dashing into the heart normally.