Items are not bound properly, they go in the grave upon death.

Edit: Tested both EnderIO and the CoFH III versions.

M4thG33k/TombManyGraves#58
SleepyTrousers/EnderIO-1.5-1.12#4709

TombManyGraves-1.12-4.1.0
tombmanygraves2api-1.12.2-1.0.0
tombmanypluginsbackpacks-1.0.0
tombmanypluginscosmeticarmorreworked-1.0.0
tombmanypluginsthebetweenlands-1.0.0

EnderIO-1.12.2-5.0.30
EnderCore-1.12.2-0.5.37

Not sure which part of this actually pertains to the Soulbound enchant.
CoFHCore-1.12.2-4.5.3.20-universal
CoFHWorld-1.12.2-1.2.0.5-universal

TombManyGraves prevents all other death processing from working by disabling the Forge API that's there for that purpose.

Replace TMG with Corail Tombstone: https://minecraft.curseforge.com/projects/corail-tombstone
Disable EnderIO Soulbound(incompatable). Potentially also disable CoFH or Corail Soulbound(redundant).