Shout out to @mihneabalta for guiding me in tracing where the issue lies:
obj refs name=/Game/MetaHumans/C1Healed01/BP_C1Healed01.BP_C1Healed01
LogReferenceChain: Display: InitialGather memory usage: 14.16
LogReferenceChain: (standalone) Blueprint /Game/MetaHumans/C1Healed01/BP_C1Healed01.BP_C1Healed01 is not currently reachable but it does have some of GARBAGE_COLLECTION_KEEPFLAGS set.
It seems that the MH blueprints are set-up to never be garbage collected - GARBAGE_COLLECTION_KEEPFLAGS set
Still investigating if there is a work-around.