Mutable references MetaHumans heads directly, but DNA Assets are private

When we switched our project to 5.6.1

This error appeared on build. The compilation of the Mutable assets in the editor is not reporting any issue.

Mutable setup with MetaHuman head assets included.

1. One Root CO with Heads group and Head component - reference for this component is SKM of Metahuman head without any DNA data.
2. CO with SKM MH head asset adding to Head component of the RCO, using RigLogic DNA node (asset defined identical as visual SKM mesh). There is more than one head linked to the Head group with the same setup.

aHumans/M_Head_002_BTA/Face/M_Head_002_BTA_FaceMesh.M_Head_002_BTA_FaceMesh:DNAAsset_7:
<pre class="ql-syntax" spellcheck="false">22:13:17  LogSavePackage: Warning:  ModelResources /Game/Art/Characters/Mutable/MCOR_M_NPC.MCOR_M_NPC:000000001F40B15AAA0E28EC914C4323Windows (1 refs)
22:13:17  LogSavePackage: Warning:    0) ObjectProperty /Script/RigLogicMutable.DNAPinData:DNAAsset
22:13:17  LogSavePackage: Warning:  SkeletalMesh /Game/MetaHumans/M_Head_002_BTA/Face/M_Head_002_BTA_FaceMesh.M_Head_002_BTA_FaceMesh (1 refs)
22:13:17  LogSavePackage: Warning:    0) ObjectProperty /Script/Engine.SkeletalMesh:AssetUserData.AssetUserData
22:13:17  LogSavePackage: Warning:
22:13:17
22:13:17  Referencers of DNAAsset /Game/MetaHumans/M_Head_001_BTA/Face/M_Head_001_BTA_FaceMesh.M_Head_001_BTA_FaceMesh:DNAAsset_7:
22:13:19  LogSavePackage: Warning:  SkeletalMesh /Game/MetaHumans/M_Head_001_BTA/Face/M_Head_001_BTA_FaceMesh.M_Head_001_BTA_FaceMesh (1 refs)
22:13:19  LogSavePackage: Warning:    0) ObjectProperty /Script/Engine.SkeletalMesh:AssetUserData.AssetUserData
22:13:19  LogSavePackage: Warning:  ModelResources /Game/Art/Characters/Mutable/MCOR_M_NPC.MCOR_M_NPC:000000001F40B15AAA0E28EC914C4323Windows (1 refs)
22:13:19  LogSavePackage: Warning:    0) ObjectProperty /Script/RigLogicMutable.DNAPinData:DNAAsset
22:14:19  LogWindows: Error: appError called: Can't save 'E:/BuildAgent/work/b3fbf36c2e0b4163/-{}/Saved/Cooked/Windows/{}-/Content/Art/Characters/Mutable/MCOR_M_NPC.uasset': Illegal reference to private object: 'DNAAsset /Game/MetaHumans/M_Head_001_BTA/Face/M_Head_001_BTA_FaceMesh.M_Head_001_BTA_FaceMesh:DNAAsset_7' referenced by '000000001F40B15AAA0E28EC914C4323Windows' (at '/Game/Art/Characters/Mutable/MCOR_M_NPC.MCOR_M_NPC') in its 'DNAAsset' property.
22:14:19  LogWindows: Warning: Failed to retrieve TPM device information
22:14:19  LogWindows: Error: begin: stack for UAT
22:14:19  LogWindows: Error: === Critical error: ===
22:14:19  LogWindows: Error:
22:14:19  LogWindows: Error: Can't save 'E:/BuildAgent/work/b3fbf36c2e0b4163/-{}/Saved/Cooked/Windows/{}-/Content/Art/Characters/Mutable/MCOR_M_NPC.uasset': Illegal reference to private object: 'DNAAsset /Game/MetaHumans/M_Head_001_BTA/Face/M_Head_001_BTA_FaceMesh.M_Head_001_BTA_FaceMesh:DNAAsset_7' referenced by '000000001F40B15AAA0E28EC914C4323Windows' (at '/Game/Art/Characters/Mutable/MCOR_M_NPC.MCOR_M_NPC') in its 'DNAAsset' property.
22:14:19  LogWindows: Error:
22:14:19  LogWindows: Error:
22:14:19  LogWindows: Error: end: stack for UAT
22:14:23  Took 1,911.19s to run UnrealEditor-Cmd.exe, ExitCode=3
22:14:23  Cook failed.
22:14:23  AutomationTool executed for 0h 36m 1s
22:14:23  AutomationTool exiting with ExitCode=25 (Error_UnknownCookFailure)
22:14:23  BUILD FAILED

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