How to reproduce:
1. Open the “IN-122895” project
2. Open the “StartScene”
3. Enter Play Mode
4. Press the letter “n” to switch to “SampleScene”
5. Expand the “SampleScene” Hierarchy and click the checkbox next to “New Sub Scene”
6. Exit Play Mode
7. Observe Scene view

Actual result: The Cube for the subscene is visible
Expected result: The Cube for the subscene is not visible

Reproducible with: 1.6.1 (6000.0.53f1), 1.10.0 (6000.0.65f1, 6000.2.15f1, 6000.3.2f1, 6000.4.0b3, 6000.5.0a4)

Reproducible on: macOS Tahoe 26.1, Windows 10 (by user)
Not reproducible on: no other environments tested

Workaround: Reloading the Scene (double-click in the Project window) removes the leftover Cube