Reproduction steps:
1. Open attached project "UIToolkitReloadBug.zip" and scene "SampleScene"
2. Enter Play Mode
3. In Game View, press the "Test" button
4. In Project window, right-click and Reimport Assets/Main.cs
5. In Game View, unpause and press the "Test" button
6. In Hierarchy window, select "Main" GameObject
7. In Inspector window, enable "Main (Script)" component's "Fix Reload" field
8. In Project window, right-click and Reimport Assets/Main.cs
9. In Game View, unpause and press the "Test" button

Expected result: The button can be pressed in all three instances
Actual result: The button cannot be pressed after initially reimporting Assets/Main.cs

Reproducible with: 1.0.0-preview.14 (2020.3.4f1, 2021.1.1f1)
Could not test with: 2018.4.34f1, 2019.4.24f1 (UI Toolkit package unavailable), 2021.2.0a12 (UI Toolkit package errors break project)