Jack doesn’t spawn neither as a whisp nor as Jack himself. On the map the correct area is marked (last pumpkin field with a few extra whisps and the seide mission with the book).
You need to trigger him by running by the pumpkin fields and I once heard that he sometimes likes to wander of but generally it SHOULD work…look around move past every pumpkin and if he still isn’t there write a petition ingame.
I walked past every pumpkin patch I found. there weren’t those big chunky extra pumpkins that look like his head anywhere. This wasn’t my first time I did the mission (more like 20 times) but the bug happened and I wanted to report it even if it’s not regularily so. I just did them for fast challenge, it’s not needed to progress or anything so no big deal for me.
I’ve noticed a couple issues with Jack’s Back.
The first is, when approaching the Ghostlight field from the direction of John Wolf (the questgiver), one of the Ghostlights - the one directly to the right of the shack, who appears on a small rise beside a bush and a tree - can be invisible. If you go up there you can sometimes bump into it (even while invisible) and then it’s red ground-circle shows up for targeting. Normally I just trigger a PBAoE beside the tree, which makes it visible.
The second is similar to the OP, though I’ve never had it occur in the final field. Sometimes I approach the Ghostlight field or the Scarecrow field, and it says to clear the field, but there is nothing there. Or, there is something there, but when I clear it, it still doesn’t trigger Jack. In those cases (possibly due to someone else having just gone through), I normally back off out of the area and wait for the field to reset.
Once or twice, when the Scarecrow field won’t reset, I’ve had to go back to the Ghostlight field, clear it all over again, which may or may not spawn another Jack there, nuke him, then go back to the Scarecrow field.
If you don’t happen to see the big lumpy pumpkins in the various patches in the final field, I’d try going back to the Revenant patch and see if there is anything you can trigger there, or even to the Scarecrow one.
Have experienced the same bug (today after reset, dim 1 server 143).
This appears to be slightly different to a phenomenon I occasionally encountered on this tier in TSW where only one of the mutant pumpkins was missing (but there was an invisible area of collision where it should be) and all the others only spawned wisps.
Eventually completed the mission by the time-honoured expedient of getting into a different instance of the zone where all the pumpkins were present.