The backpack deletes anything inside of it when you die with keep inventory on

Looks like this can be fixed by adding if(this.player.getSleepTimer() == 0) return; right under if (slot == 38 && !this.player.world.isClient) { inside MixinPlayerInventory.java. However this is some duct tape and bubble gum kind of patching up and I still have to test it for a longer period of time...
But until the authors fix it, this might help someone who desperately needs it.