How to reproduce it:
1. Open the user's attached project 'PlunderBuns.zip'
2. Load 'EncounterScene' scene from 'Assets' > 'Scenes' folder
3. Enter the Play Mode
4. Click on the 'Cannon' button on the top of the Game view
5. Move the mouse around the Game view
6. Observe the cursor

Expected result: The customized cursor doesn't change after moving out of the 'Cannon' button
Actual result: The customized cursor changes back to default after moving out of the 'Cannon' button

Reproducible with: 1.0.0-preview.9 (2020.1.3f1, 2020.1.15f1), 1.0.0-preview.12 (2020.1.15f1, 2020.2.0b13, 2021.1.0a5)
Not reproducible with: 1.0.0-preview.3 (2020.1.0b12), 1.0.0-preview.6 (2020.1.2f1)
Could not test with: 1.0.0-preview.7, 1.0.0-preview.8 (2020.1.3f1) (due to the TextCore missing reference errors), 1.0.0-preview.12 (2021.1.0a6, 2021.1.0a10) (due to internal errors)

Notes:
- Also reproducible on Windows Standalone build
- UI Toolkit was not yet introduced: 2018.4, 2019.4, 2020.1