Right clicking my grave will often, but not every time, delete any item in the first action bar slot. This includes items retrieved from the grave, typically my ethereal scythe from enigmatic legacy, but it will delete any item from what I can tell.

I'm running Monumental Experience pack, version 17.0.7 and EnigmagicGraves 1.16.5-1.7.4-hf2 in single player.
OP is on, not sure what other relevant details you need.

Issue also occurs on EnigmaticGraves 1.18.1-1.7.4

Good-bye to my enchanted sword :(

Hmm can you tell me any more info about when it happens? Maybe the items are enchanted with something odd, like soulbind of sorts. Was your inv full when you picked up or retrieved. Did you have another item of the same type as the one that disappeared (like a second diamond sword for example, even if the enchants arent the same) when it happened?

Similar issues have been at it for a while but I never manage to get enough info when it happens, also weirdly for just one item.

I guess any server logs might come in handy if anyone has any

Right clicking my grave

It happens when I recover items from a grave. My stuff wasn't enchanted but only the first slot on the hotbar is affected.
It will delete an item recovered from a grave and/or items that were in the inventory prior to grave recovery.
Occurs with inventory space available.

I don't have logs from 6 months ago,

Did it only happen with a non-stackable item, or do you recall with stackables too?

I don't think I tried with stackable items.