Reproduction steps:
1. Create a new project
2. Create a GameObject (e.g. Cube)
3. Set it as Default Parent
4. Enter Play mode
5. Observe the GameObject in the Hierarchy

Actual result: GameObject is no longer a Default Parent
Expected result: Nothing of note happens

Reproducible with: 6000.3.0a1, 6000.3.0b3, 6000.4.0a1
Not reproducible with: 2021.3.56f1, 2022.3.67f1, 6000.0.58f1, 6000.2.0b2, 6000.2.6f1

Reproduced on: macOS Sequoia 15.4 (M4), Windows 11