How to reproduce:

1. Open attached project "843499-repro.zip"
2. Open "test" scene
3. Select Main Camera
4. Try to delete elements individually from the array via Inspector

Expected behavior: It's possible to delete an array element with delete key or right click -> delete
Actual behavior: The only way to delete an array element is by reducing the size of the array

Note: Everything works as expected without Custom Property Drawer on the same MonoBehaviour

Reproducible: 5.3.7p2, 5.4.3p2, 5.5.0f3, 5.6.0a5