This may not be considered a bug. But I'm not sure.
By default, a traveler's backpack which the player is wearing will be placed where the player dies as the death urn is. Therefore the death urn will disappear and everything in it scattered around. If I set the config "Place Backpack on Death" to false, the death urn will work, but the backpack will not be reserved by the urn, it will drop. I think it is because the backpack that's being worn has no specific slot and the death urn doesn't consider it part of the player's inventory. (But ./gamerule keepInventory true works on the backpack, so maybe death urn should work too? )
As I said, this is not necessarily a bug. If you do not want to fix it, could you please add relative coordinates where the death urn spawns to the config file? In this way the player can change them to 0 1 0 so that the death urn spawns 1 block higher than traveler's backpack to avoid the conflict.

Version
EnderCore: 1.12.2-0.5.76
EnderIO: 1.12.2-5.3.70
TravelersBackpack: 1.12.2-1.0.35
Minecraft: 1.12.2
Forge: 14.23.5.2854
Single Player