I have this issue in my households all the time. (I frequently save elderly sims from death with the death flower) A sim "spared" is actually killed and then resurrected. The issue is that if a sim doesn't witness the "resurrection" the game doesn't clear the mourning moodlet. So, if the sim is sleeping, or at work, or off-lot, or doing another action in their queue that doesn't get interrupted (like a programming task on the computer) - the game will queue up the death and give them the mourning moodlet, but it doesn't queue up any way of witnessing the resurrection and removing the moodlet.
Definitely a bug, and the entire process takes so long it's mind-numbing to micro-manage everyone in the household to avoid it. You have to keep everyone in the household around Grim and the deceased, after accepting a death flower or successful pleading, Grim will usually still do his reaper killing with the scythe, then take a while to check his notes, queues up a resurrection action and the dead sim rises up and is resurrected. You have to keep all of the witnesses in the area for the entire process (2 or 3 sim-hours) and wait for them to each give a "hooray" action and get a thought bubble with a picture of the resurrected sim over their heads.
What I have found is that the best way to handle this is to immediately clear the queues of everyone present, wake up anyone who is sleeping and form a group with everyone in the household. Keep clicking a spot next to the deceased / Grim and choose "Go Here" from the group actions. They will frequently start to wander away as soon as the dead sim gets reaped, before Grim starts to resurrect them. Watch carefully, you want to see the mourning moodlet appear on each one and then wait for the resurrection and keep them there to queue up the "See What's Happening" task to witness the resurrection and remove the moodlet. Then, if there are any gravestones or urns on the lot, most of them will race over and quickly start mourning someone else to add another mourning moodlet, so you'll want to pause the game and queue other interactions right after.
If someone's at work, it's about a 50/50 chance that they'll have the mourning moodlet when they get home. I have found the best way to deal with the mourning moodlets is to have them buy a moodlet solver and drink it. It removes all of the negative moodlets including the mourning moodlets. If you have toddlers in the mix it's really challenging; they will inevitably have their needs run down to red by the time the whole process is over and they can't drink the potions.
I would love to see this bug get some attention. It's not all that horrible if it happens once in a while, but if you're running an elderly sim that you want to keep alive for a while longer you're basically spending half a day just micromanaging the death process like this.
