Reproduction steps:
1. Open the attached project “CallbackBug”
2. Open the scene “Assets/Scenes/SampleScene”
3. In the Hierarchy window select the GameObject called “GameObject”
4. In the Inspector window add the list item to the “Unity Event ()” list
5. Add the script of your choice to the newly created list item
6. Assign the function to the list item

Expected result: Callback is registered when assigning the function
Actual result: The change is not detected

Reproducible with: 2021.3.19f1, 2022.1.0a15
Not reproducible with: 2022.2.8f1, 2023.1.0b7, 2023.2.0a5
Couldn't test with: 2020.3.45f1 ('InspectorElement' does not contain a definition for 'FillDefaultInspector')
Fixed in: 2022.1.0a16

Reproducible on: Windows 11