I'm just going to paste the comment from curseforge here:

I normally use forgotten graves on fabric but I have used others and never saw that any of them actually keep the backslots items, they're just deleted. I have used Forgotten Graves and You're in Grave Danger. I haven't used the second one since maybe 1.16.5 so there may have been an update to make it compatible, but I don't really know how to check for features released after a specific version other than clicking through every update the mod's had since then. Hopefully this can be fixed your side so I don't have to message a few backpack mod developers and try to get one to make it compatible with the backslot mods. Thank you for your help!

And I might've also used Corail Tombstone at one point but not sure if it was with these mods.

Also same with Backslot addon. Gravestone mods don't keep items in that slot either

Should be compatible with the latest version.
Basically this provides the info

You're in grave danger has compatability with backslot, except for when BackSlots "lifesaver"-function activates because of other mods, as mentioned in #84 for example.