Lithium Sunset Heart?

[Spartanburg123]

Marsha sent me seeds early in 2017. I grew Lithium Sunset- beautiful striped beefsteaks, red and yellow stripes, with excellent flavor. I gave a few seedlings to my coworker Mike, who provided a few seedlings to a friend. He observed at least three tomatoes on his plant that were blunt hearts, while the others were oblate. He saved those seeds from the affected fruit. Besides enviromental factors, is there anything else that could be going on? I'm not familiar with any striped heart tomatoes.....Thanks!

Darin