I've only had it happen it happen with vampires. That does not mean it won't happen with other sims; I've just not seen it happen.
This happened to me too, maybe 6m-1y ago. Not sure of the timeline, but I'm still playing the particular sim's family for 4 generations now and she still has her own grave in her inventory. I connected it with my savescumming her death. At the time there was an issue with vampires hanging around autonomously outside, which in my observation has been fixed. I can't get rid of the urn, it cannot be sold or deleted. Placing it gives almost no options, as she is not really dead, besides it having her name on it. Sorry this comes so late, it didn't effect me enough to research the problem, but I just happened to see this searching for a separate issue.
