The scoreboard objective "stat.useItem.minecraft.lit_pumpkin" does not track every time a player places a Jack o'Latern. it should work in the same way that stat.useItem.minecraft.stone tracks players placing stone blocks but it doesn't.

In fact the scoreboard objective "stat.useItem.minecraft.pumpkin" tracks both placing pumpkins and placing Jack o'laterns which is clearly unintended behaviour.