Hiya!

So I seem to have lost Herb Oldie's grave. I think Michael Bachelor's urn too but I'm not as fussed about that. I have all the maty mods and everything and know exactly how to get them back as this is by far not the first time I've had graves go poof. So that's not what this is about.

But when I google it it's always like tutorials on how to get them back, and so I'm not sure if anyone in the sims community has figured out exactly why it happens or a definitive way to prevent it?

In my case here I'm wondering if it has something to do with the move to community lot option, if there's something wrong with that. I also think there's just something wrong with that particular community lot in general, it's a custom church & cemetery I downloaded but although set as a community lot when I placed it there was no telephone so my sims were stuck. I've since added a phone myself but I wonder if the lot is just wonky to start. This could have nothing to do with it.
But I'm more wondering if the move to community lot option is wonky because Dirk still has mama Darlene's grave in his backyard under that tree just fine. It's just Herb and Michael who I had tried to move who aren't at the church/cemetery anymore.

If I'm going to be honest, I'm unsure if there is a definitive cause, nevermind a method to prevent it.

It just appears to be a glitch that happens without rhyme or reason, as far as I know anyway.

As for graves that go missing when travelling to a community lot, I found a post that Mootilda made in 2014 that explained it pretty well:

HoodChecker can be used to retrieve them.

I'm not entirely sure either why graves will vanish or go missing, but as CrystalFlame360 already said you can use HoodChecker to retrieve any graves stuck in transit. You can also use the batbox FFS Lot Debugger to respawn missing gravestones. You'll need to have TestingCheats enabled, be able to teleport sims in the "Default" family to your current lot, and be able to make the dead sims selectable somehow, method is up to you (I use the Insimenator summoner for this). Once the dead sim has been summoned and made selectable, SHIFT-click on the batbox and choose Items > Respawn Tombstone. Note that doing it via this method will automatically assign any elder sims' death types as old age and will give the platinum tombstone type if they have the necessary aspiration level. You might want to SHIFT-click the new grave and make sure the death type is correct.

Alternatively, there is this Urnstone Spawner which is quicker than the above method, but I haven't personally used it so I can't verify if it works properly or not. If you do the manual method, be careful not to accidentally summon any universal NPCs that are also in the "Default" family, like the grim reaper or hula dancers.

Yeah, no idea. I've seen people say it happens if you save while ghosts are out and about, but I've had tombstones vanish in the middle of the day, or on lots that I hadn't even got around to playing. I used to use the Batbox to respawn them, but Michael Bachelor's urn flatly refused to spawn so I wound up using that Urnstone Spawner, which I'd already downloaded to repopulate the Goth graveyards because I was sick of all the clicking I needed to do to get them back.