Reproduction steps:
1. Open user's project "UI Toolkit Bug Repro.zip"
2. Open "SampleScene" Scene
3. Click on "Parent1" in the Hierarchy window and look at the Inspector window
4. Change Sorting Layer of "Control Child Settings" Script from "Default" to "Foreground"
5. Notice how child GameObject (red circle) doesn't come to the foreground
6. Change Sorting Layer of "Control Child Settings" Script from "Foreground" to "Background"
7. Notice how child GameObject (red circle) comes to the foreground

Expected result: Upon change of parent GameObject's Sorting Layer, child GameObjects get updated to the same settings as parent GameObject
Actual result: Upon change of parent GameObject's Sorting Layer, child GameObjects get updated to the old settings of parent GameObject

Reproducible with: 2019.4.23f1, 2020.3.2f1, 2021.1.1f1, 2021.2.0a11
Couldn't test with: 2018.4.33f1 (couldn't downgrade to a functioning project)

Notes:
1) Changing Color of "Control Child Settings" Script has the same issue
2) There are additional debug messages in the console after the 4th and 6th steps that show how settings get applied