You need to clone the properties on death because technically when you respawn you have a different EntityPlayer instance and so your previous extended properties would get lost.
For player entities, capabilities attached to them will not persist across death and respawn. To copy them, you will need to handle the PlayerEvent.Clone event. For example, something like this in the event handler:
// clone capabilities
EntityPlayer player = event.getEntityPlayer();
IMana mana = player.getCapability(ManaProvider.MANA, null);
IMana oldMana = event.getOriginal().getCapability(ManaProvider.MANA, null);