I would make the heart in Photoshop. Black heart on white background. Make it big enough so that a 720X480 frame fits into the black area.
Then in Premiere Pro, scale it down to 0 and scale it back up with keyframes. Do this in a new sequence. Then nest that sequence into the main sequence and use it as a track matte. Since the scaling happens in a different sequence, it should solve your problem.
Steven
https://www.stevengotz.com