How to reproduce:
1. Open the attached project “FBXPrefab.zip“
2. Open the Prefab “MyPrefabVariant“ Hierarchy (Assets)
3. Attach the “TestPrefab01“ Prefab (Assets/Prefabs) to the “arms_AuxSHJoint” child GameObject
4. Exit the “MyPrefabVariant“ Prefab Hierarchy
5. Select the “mdl_runaway“ FBX file (Assets/runOp)
6. Enable “OptimizeGameObjects“ and hit “Apply“
Expected result: The attached child Prefab “TestPrefab01“ exists in the “MyPrefabVariant“ Prefab Hierarchy
Actual result: The attached child Prefab “TestPrefab01“ doesn’t exist in the “MyPrefabVariant“ Prefab Hierarchy
Reproducible with: 2020.3.40f1, 2021.3.11f1, 2022.1.17f1, 2022.2.0b8, 2023.1.0a12
Reproduced on: macOS (12.5), Windows 10
Note: When trying to reproduce the issue, if the 4th step is skipped, the “TestPrefab01“ Prefab doesn’t disappear, but detaches from the parent GameObject