Steps to reproduce:

Open the "ET66.zip" project
Open the "SampleScene"
Select the Panel Settings in the Project and set the Render Mode to "Screen Space Overlay"
Open Scene view
Edit any in-scene GameObject's Transform in any way
Select the Panel Settings again and in Inspector, change the Render Mode to "World Space"
Observe the Scene view

Actual results: The World Space UI doesn't get displayed right away

Expected results: The World Space UI is displayed right away

Reproducible with versions: 6000.6.0a6, 6000.6.0a7

Not reproducible with versions: 6000.0.77f1, 6000.3.18f1, 6000.4.12f1, 6000.5.0f1, 6000.6.0a5

Tested on (OS): Windows 11 Pro (25H2)

Workaround: Change any property of any GameObject in the Scene

Notes:

Any change to any component in the SampleScene (or saving the scene) will update the UI to display in World Space
Entering Play Mode will start displaying the WorldSpace UI
It appears that the regression happens only when a Theme Style Sheet is assigned to the Panel Settings