"Would that make this an incomplete or unstable cross?"

No. It simply means what it says. Inconsistent shape doesn't mean the variety isn't "true". Shape is only one trait for defining stability and is often insistent due to other variables. So it is a question of "majority". Given good conditions, a heart produces mostly heart shaped fruit.

Even the oldest, most stable hybrid varieties will produce inconsistently shaped fruit as it depends on a large number of other variables - conditions at the time of pollination, degree of pollination, source and age of the pollen, ambient conditions during growth, nutrient demands and distribution, etc. etc.

"What should one do, keep saving seeds from the heart shaped fruits over generations and hope they eventually come true?"

That is called "selecting" (selection for trait) not crossing. Sure you can do as you describe but given all the other variables it still won't insure consistent shape production. Selection can increase shape consistency in the % of fruit produced IF all other variables are consistent as well and without a totally controlled environment that is impossible.

Dave