i just tried killing an orc on another map, that worked and spawned a grave,,
the error img you posted is enemy related, on actors it should look slightly different..
also the first line of th error is about this:
View attachment 207435
View attachment 207436
which makes no sence to me at the moment..
getting the x-position_data of any map event should not make bugs..
also it should not make any difference on which map Units get killed,
it should work on all maps aslong there is a grave on the grave map ,
that fits to each actorID and enemyID which is used on your battlemap..
i will take some time later to look over the whole code again, but at the moment i got no idea..