I did not test this outside the custom venue and it is one of the roles for the custom venue. I have ghosts in this kind of cemetry/crematorium to spook a little the visitors. This is the first time reionizing this, by my search about it I found other people talking about ghost problems as example in the reaper career are ghosts for someone as humans too.
Yeah there's several causes which I'm wondering if this particular one is showing as ghost outside of custom venues.
There does seem to be a main issue where Sims outside of the household don't become proper ghosts but it can also have to do with missing death types, especially for non-adult Sims.
Can you switch to the household and see what he supposingly died of ? Either try on a backup or don't save your game.
