Use of "j" is strictly a question of orthography. The Romans themselves did not have a "j" in their alphabet. It was a later invention that was used in some eras to represent "consonantal i," such as the initial i of iacta or iuvat. Modern texts almost universally eschew it.

So "jacta" isn't really "wrong," it's just out of date. Of course, I'm one of those holdfasts who would like to see the "j" preserved. On some days, I'm even tempted to write "quum" in all my books for "cum coniunctivum."