UI Builder "Filter" foldout collapses when entering Play mode or docking the window

Steps to reproduce:

1. Create a new project using Universal 3D template
2. Go to Assets > Create > UI Toolkit > UI Document (UXML)
3. Open the created .uxml asset in UI Builder
4. Add a Visual Element to the Hierarchy
5. In the Hierarchy, expand the "Filter" foldout
6. Enter Play mode
7. Observe the "Filter" foldout

Actual results: Foldout collapses

Expected results: Foldout remains expanded/open

Reproducible with versions: 6000.3.8f1, 6000.4.1f1, 6000.5.0b1, 6000.6.0a1

Can’t test with versions: 6000.0.71 (Filter foldout not available)

Tested on (OS): macOS 26.3

Notes:

• Also reproduces when docking/undocking the UI Builder
• Does not reproduce with other foldouts
• Does not reproduce if all foldouts are expanded