Nested PanelRenderer never receives its initial UIReloadCallback when entering Play mode

How to reproduce:

1. Open the attached "IN-150082" project
2. Open Assets > Scenes > SampleScene scene
3. Enter Play mode and wait about 4 seconds
4. Observe the Game View and the Console

Actual result: Only the top-level PanelRenderer ("UIParent") receives a UIReloadCallback and renders. The nested child ("UIChild") never does, so its UI is absent, with no error or warning
Expected result: Both PanelRenderers receive a UIReloadCallback and render their content

Reproducible with: 6000.6.0a7, 6000.6.2f1, 6000.7.0b1
Not reproducible with: 6000.6.0a6
Could not test with: 6000.0.84f1, 6000.3.24f1, 6000.5.0a3 (PanelRenderer feature appeared in 6000.5.0a4), 7000.0.0a1 (Editor crashes on entering Play mode)

Reproducible on: Windows 11 (25H2)
Not reproducible on: No other environments tested