Reproduction steps:
1. Open user's attached project
2. Open scene "SampleScene"
3. From the Project window, open " MyAuthoringBehaviour.cs"
4. Change the namespace, e.g. by adding ".test" at the end of it
5. Increase the ConverterVersion by one
6. Go back to the editor and look at the Entity information in the Entity Debugger, or observe Logs\AssetImportWorker0.log

Expected result: Entities are converted with the "MyData" Component attached
Actual result: "MyData" Component is not attached, no warnings can be seen in the editor (except 2021.1 where a warning is thrown)

Reproducible with: 0.14.0-preview.19 (2020.1.11f1, 2020.2.0b9, 2021.1.0a3)
Not reproducible with: 0.14.0-preview.19 (2021.1.0a4)
Could not test with: 0.11.2-preview.1 (2018.4.28f1, 2019.4.13f1 because of errors)

Note:
-In older versions than 2021.1, this error is printed in the "AssetImportWorker0.log":
The referenced script is missing on GameObject
-In 2021.1, this error is throw in the editor:
[Worker0] The referenced script is missing on GameObject

Workaround:
Changing the Namespace, restarting the editor, and increasing the ConverterVersion