Are you seeing this mainly near the tail or down the whole length?
Note that the two Burmese pattern mutations show sporadically in hets. Some het green and granites look perfectly normal while some are the recognizable leopard and puzzle hets types. It will be interesting to see if this can be proven to be a reliable indicator in the ball python piebald pattern mutation but even if it does work out I would expect that some hets will not show it. I don't have a clue on the mechanism by which some pattern hets show and some don't.
I’m not saying that all pattern mutations will sometimes display in hets, just that there is a precedence for some of them to show some of the time.
My sister bought three 50% possible het piebald males from Ralph Davis (produced by Peter Kahl) and one of them has a fair amount of this lining. His only daughter does not. However, two of the three daughters of one of the ones that doesn't have the lining have some of this toward the tail.