Reproduction steps:
1. Create a new project
2. Install Unity Physics package (Window > Package Management > Package Manager)
3. Create new Sub Scene (New Sub Scene > Empty Scene…)
4. Create any GameObject inside the Sub Scene
5. Create a child GameObject for the newly created GameObject
6. Drag the child GameObject out to remove its child status
7. Observe the Console

Actual result: "Failed to find Entity child node" error is thrown
Expected result: No warnings or errors are thrown

Reproducible with: 6.5.0 (6000.5.0b3, 6000.5.3f1, 6000.6.0b3, 6000.7.0a2)
Not reproducible with: 1.4.6 (6000.0.79f1, 6000.3.19f1), 6.5.0 (6000.5.0a8)
Couldn’t reproduce with: 6.5.0 (6000.5.0a9-6000.5.0b2) (Due to another bug)

Reproduced on: macOS Tahoe 26.5 (M4), Windows 11

Note:

FAV is in Editor versions
Issue is not reproducible if the child GameObject is created separately and then dragged into the parent
Full error:

Failed to find Entity child node: Entity(0:3) in HierarchyEntityHandler mapping
UnityEngine.Debug:LogError (object)
Unity.Debug:LogError (object) (at ./Library/PackageCache/com.unity.entities@0c17b01e5ab1/Unity.Entities/Stubs/Unity/Debug.cs:11)
Unity.Entities.Editor.HierarchyEntityHandler:SetParentNode (Unity.Entities.World,Unity.Collections.NativeList`1<Unity.Entities.Entity>,Unity.Collections.NativeList`1<Unity.Entities.Entity>) (at ./Library/PackageCache/com.unity.entities@0c17b01e5ab1/Unity.Entities.Editor/Hierarchy/HierarchyEntityHandler.cs:270)
Unity.Entities.Editor.UpdateHierarchySystem:OnUpdate () (at ./Library/PackageCache/com.unity.entities@0c17b01e5ab1/Unity.Entities.Editor/Hierarchy/UpdateHierarchySystem.cs:293)
Unity.Entities.SystemBase:Update () (at ./Library/PackageCache/com.unity.entities@0c17b01e5ab1/Unity.Entities/SystemBase.cs:205)
Unity.Entities.ComponentSystemGroup:UpdateAllSystems () (at ./Library/PackageCache/com.unity.entities@0c17b01e5ab1/Unity.Entities/ComponentSystemGroup.cs:735)
Unity.Entities.ComponentSystemGroup:OnUpdate () (at ./Library/PackageCache/com.unity.entities@0c17b01e5ab1/Unity.Entities/ComponentSystemGroup.cs:688)
Unity.Entities.SystemBase:Update () (at ./Library/PackageCache/com.unity.entities@0c17b01e5ab1/Unity.Entities/SystemBase.cs:205)
Unity.Entities.ScriptBehaviourUpdateOrder/DummyDelegateWrapper:TriggerUpdate () (at ./Library/PackageCache/com.unity.entities@0c17b01e5ab1/Unity.Entities/ScriptBehaviourUpdateOrder.cs:523)

Only reproducible with the New Hierarchy