As mentioned in comments I've got some sort of mod conflict that's causing the "Create Corpse Child" job to create a Corpse Child, as a random new pawn, and not consume the corpse. Here's the exception:

Originally posted by Me:

JobDriver threw exception in initAction for pawn Skeleton driver=JobDriver_ConstructCorpseChild (toilIndex=2) driver.job=(CorpseChildren_CreateCorpseChild_JobDef (Job_254487) A=Thing_Corpse_Human28381)
System.Collections.Generic.KeyNotFoundException: The given key was not present in the dictionary.
at System.Collections.Generic.Dictionary`2[TKey,TValue].get_Item (TKey key) [0x0001e] in <eae584ce26bc40229c1b1aa476bfa589>:0
at (wrapper dynamic-method) CorpseChildren.JobDriver_ConstructCorpseChild.CorpseChildren.JobDriver_ConstructCorpseChild.<MakeNewToils>b__8_1_Patch0(CorpseChildren.JobDriver_ConstructCorpseChild)
at (wrapper dynamic-method) Verse.AI.JobDriver.Verse.AI.JobDriver.TryActuallyStartNextToil_Patch0(Verse.AI.JobDriver)
UnityEngine.StackTraceUtility:ExtractStackTrace ()
Verse.Log:Warning (string)
(wrapper dynamic-method) Verse.AI.JobUtility:Verse.AI.JobUtility.TryStartErrorRecoverJob_Patch1 (Verse.Pawn,string,System.Exception,Verse.AI.JobDriver)
(wrapper dynamic-method) Verse.AI.JobDriver:Verse.AI.JobDriver.TryActuallyStartNextToil_Patch0 (Verse.AI.JobDriver)
Verse.AI.JobDriver:ReadyForNextToil ()
(wrapper dynamic-method) Verse.AI.JobDriver:Verse.AI.JobDriver.DriverTick_Patch0 (Verse.AI.JobDriver)
(wrapper dynamic-method) Verse.AI.Pawn_JobTracker:Verse.AI.Pawn_JobTracker.JobTrackerTick_Patch1 (Verse.AI.Pawn_JobTracker)
(wrapper dynamic-method) Verse.Pawn:Verse.Pawn.Tick_Patch2 (Verse.Pawn)
(wrapper dynamic-method) Verse.TickList:Verse.TickList.Tick_Patch0 (Verse.TickList)
(wrapper dynamic-method) Verse.TickManager:Verse.TickManager.DoSingleTick_Patch7 (Verse.TickManager)
Verse.TickManager:TickManagerUpdate ()
(wrapper dynamic-method) Verse.Game:Verse.Game.UpdatePlay_Patch1 (Verse.Game)
(wrapper dynamic-method) Verse.Root_Play:Verse.Root_Play.Update_Patch1 (Verse.Root_Play)

Of note, I decided to start completely fresh in order to just test with the scenario forcing the background instead of forcing the trait and background myself *because* I couldn't find the background in Character Edit.

Turns out the scenaria "Force background" also can't find the background, so potentially something is preventing the background from being loaded? Or I have a corrupted mod, so I'm gonna unsubscribe and resubscribe and see if the background reappears xD

::EDIT::
So it was likely because of Just Better Pawn Generation, as one of the requirements for that is this backstories implementer
https://steamhost.cn/steamcommunity_com/sharedfiles/filedetails/?id=2024991575

disabling JBPG and all it's prereqs including the above mod and testing like that.

::EDIT 2::
Yep, that appears to be the conflict, most likely due to the backstory loader needing to have the backstory manually added. I'll just avoid using them tbh, I prefer to be able to do the Corpse Child stuff properly rather then have the pawn generation system they use.