How to reproduce, option 1, if you are comfortable getting a Unity project running from the attached zip folder:

1. Open the attached “Bugreport.zip“ project
2. Build and Run the “AR_Main“ and “2D_Main“ Scenes for Android
3. See that the elements of UI Toolkit are displayed
4. Press the “Switch to: World Camera“ button
5. Observe the Scene

How to reproduce, option 2, if you already have ARFoundation-Samples cloned on your computer, or if it is just easier than trying to make the Bugreport.zip project run (which was made on an old version of Unity). The branch "origin/tmp/arfb-380-ui-toolkit-built-in-pipeline" contains a project set up with UI toolkit elements already added to the "SimpleAR" and "FaceTracking/FaceMesh" scenes, with the Universal Render Pipeline package removed, and the built-in renderer enabled.

1. Checkout the "origin/tmp/arfb-380-ui-toolkit-built-in-pipeline" branch.
2. Build ARFoundation by running the "bee" files in a terminal/powershell window in the folders "arfoundation\com.unity.xr.arfoundation\Source~\XRSimulation" and "arfoundation\com.unity.xr.arcore\Source~", then open "arfoundation\samples" in the default Unity version in ProjectVersion.txt.
3. Build and Run the sample app on Android, then open the "FaceTracking/FaceMesh" scene and observe.

Expected result: Elements of UI Toolkit are displayed
Actual result: Elements of UI Toolkit aren’t displayed (reference attached screenshots)

Reproducible with: 4.2.7, 5.0.3 (2021.3.16f1), 5.0.3 (2022.2.2f1, 2023.1.0a25), 6.0.0 (2023.3+)
Couldn’t test with: 5.0.3 (2020.3.44f1) (AR Foundation Package errors after downgrade)

reproducible with these devices:
Samsung Galaxy Z Flip3 5G (SM-F711B), Android 12, CPU: Snapdragon 888, GPU: Adreno (TM) 660
Samsung Galaxy S22 (SM-S901B), Android 12, CPU: Exynos 2200, GPU: Ltd.
Samsung Galaxy S10+ (SM-G975F), Android 10, CPU: NOT FOUND, GPU: Mali-G76
Samsung Galaxy S21 Ultra (SM-G998U), Android 13

Not reproducible with these devices:
iOS (user’s information)

Couldn’t test with these devices (the camera didn’t change facing direction):
VLNQA00494 - iPhone 14 Pro Max, 16.0.3 iOS
VLNQA00358 - iPhone 12, 14.1 iOS
VLNQA00392 - iPad (9th generation), 15.0 iOS
VLNQA00310 - iPad Pro 12.9", 13.4.1 iOS

Note: You can still press buttons, even when they aren't visible