Reproduction steps:
1. Open user's project "UI Toolkit Modal.zip"
2. Click on Test/Main Window in the top toolbar
3. Click on the "Show Modal" button to open the Modal Window
4. Wait for a few seconds and press the "Close" button

Expected result: Scheduled items, asynchronous tasks, and Update() start running upon opening of the Modal Window
Actual result: Scheduled items, asynchronous tasks, and Update() don't start running open opening of the Modal window and only start running after closing the window (only async task runs)

Reproducible with: 2019.4.26f1, 2020.3.8f1, 2021.1.7f1, 2021.2.0a17
Couldn't test with: 2018.4 (Failed to downgrade to a functioning project)

Notes:
- If you comment out the 15th line "window.ShowModalUtility()" in the "ModalWindow.cs" script expected results are observed
- Issue is also observed in Mac OS