How to reproduce:
1. Open the attached “IN-135364” project
2. Open the “SampleScene”
3. Open the Preferences window (Unity > Settings mac] )
4. Make sure “Use new Hierarchy window” is toggled on
5. In the Hierarchy, expand all the GameObjects
6. Select any child GameObject
7. On your keyboard, press the left arrow key
8. Observe the Hierarchy

Actual result: Nothing happens, parent is not collapsed and the children are still visible
Expected result: Parent is collapsed and children are hidden

Reproducible with: 6000.3.0a2, 6000.3.10f1, 6000.4.0b11, 6000.5.0a8
Couldn’t test with: 6000.0.69f1, 6000.3.0a1 (New Hierarchy not implemented)

Reproducible on: Windows 11, macOS 26.3 (M1 Pro), Windows 10 (user)
Not reproducible on: No other environment tested

Notes:
- If the parent is selected instead and left arrow is pressed, the parent will collapse
- This behavior was present in the old Hierarchy