Ever since the latest update, I'm noticing a bug. This could be another mod but the timing makes me think it's definitely SimplySwords.

Whenever someone dies, they will (inconsistently) leave no death message, and won't leave a grave with You're In Grave Danger. Nothing is left in server logs when this happens, and it's like the server doesn't even detect their death.

I would need more information. What weapon are they using, what ability, gems, runic powers.

Simply Swords only modifies behaviour when a weapon is equipped. It does not contain any mixins that override vanilla Minecraft behaviours, so this could only be a Simply Swords issue if it's relating to an item from the mod.

It seems to be present with any simplyswords weapon or runic combination, regardless of how the kill is done, as long as it's done by a player.

I think it might be another mod?
I can't think of any reason this would occur, other than the Immolation effect.
I will try to reproduce this, but I already tested Simply Swords with YIGD before I pushed this most recent release and saw no issues.

It was possibly due to a packet issue since it hasn't occurred since I added XL Packets. If that's the case, I seriously doubt it was this mod. Thanks though!

Thanks for letting me know!
I was unable to reproduce the issue in-dev and in our SMP, so that clears things up.