i went at the correct place, found the monster (it was a graboid) but when i killed it, the mission did not complete.
after a few tries i changed the “target_id” : “” value of the savegame to: “target_id” : "mon_graboid"
At loading there was a debug error (i guess id must be a number? did not check), but the monster succesfully spawned, and it was named (in all the other tries it had no name). Killing it this time progressed the quest correctly.

[tt]target_id[/tt] is for the target overmap terrain, it has nothing to do with monsters.

The mission spawns two graboid monsters, only the second one (which is named) must be killed (the other is not linked to the mission).

I also found (through the jabberwock mission) that new monsters might not be put into the reality bubble if the mission is placed near the player. In my case the starting NPC gave the jabberwock mission, the monster was created on a forest square adjacent to the starting shelter. The jabberwock was not there, but did appear after debug teleporting away and back.

ah, i see. no bug then, just the second graboid was shy.

Sounds like there is a bug–target critters not always getting spawned properly. Thanks for reporting.