Reproduction steps:
1. Open user's attached project "Bug Report - C4D scripted importer"
2. Open the Console, no "I have been called" log is printed
3. In the Project, right-click on "empty_test_file" and press Reimport
4. Open Console again

Expected result: ScriptedImporter is called after opening the project and after a reimport
Actual result: ScriptedImporter is not called after opening the project

Reproducible with: 2019.3.0f2, 2020.1.0a15
Could not test with: 2017.4.35f1, 2018.4.14f1, 2019.2.15f1 (errors)

Note1: "Unity no longer supports importing Cinema 4D files directly, but we're working with Maxon to make the experience better. To import Cinema 4D files, install Maxon's external importer. You can find more information on the Unity Forum: https://on.unity.com/c4d." warnings are shown after opening the project

Note2: Rarely(1/15), the ScriptedImporter is called after opening the project