OnClick event can use private method when that method is assigned before being made private

Reproduction steps:
1. Open the attached project "BugRepro.zip"
2. Open "SmpleScene" Scene
3. Enter Play mode and click on the button in the Scene
4. Observe the Console

Expected result: Nothing or errors are printed in the Console
Actual result: "Testing" is printed in the Console

Reproducible with: 2020.3.36f1, 2021.3.5f1, 2022.1.7f1, 2022.2.0a18, 2023.1.0a3

Notes:

• Only public method can be added to OnClick event
• Method that is already added and its access specifier is changed to private/protected is not removed and is usable by the event