UE 5.5.4 Corrupts UE4 Mannequin Skeletal Mesh on Import

Development Character & Animation
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When importing a UE4 Skeletal Mesh (created in Blender), UE5 does not recognize the root bone. This caused the “merge bones?” popup to appear on import. I ended up with a root bone named, “root.001”.

Restarting the editor fixed the issue, and the root bone is properly named “root”. I was also able to successfully import the same skeletal mesh, with no problem, into a 4.27 project and a 5.4 project.

Steps to Reproduce:

  1. Model, rig, and weight-paint a humanoid skeletal mesh in Blender 4.0 (I used Game Rig Tools).
  2. Import the skeletal mesh into UE 5.5.4. Select the UE4 Mannequin.
  3. When prompted with an error, do not merge the bones.
  4. Preview mesh with an animation.

Result:
The editor generates errors:

“This skeletal mesh asset has an incompatible Skeleton. Assign a compatible skeleton to this skeletal mesh asset.”

“LoadErrors: The Skeleton UE4_Mannequin_Skeleton is missing bones that SkeletalMesh mySkeletalMesh needs. They will be added now. Please save the Skeleton!”

“Warning: Can’t save ‘myFilePath/mySkeletalMesh.uasset’: Illegal reference to private object: ‘FbxSkeletalMeshImportData /Game/myDirectory/mySkeletalMesh.mySkeletalMesh:FbxSkeletalMeshImportData_0’ referenced by ‘mySkeletalMesh’ (at ‘/Game/myDirectory/mySkeletalMesh’) in its ‘AssetImportData’ property.”