Thanks for letting us know. Unfortuantely you've discovered a bug where under some conditions it does not show a warning.
If you add the field using AnnotatedPluginDocument.setFieldValue then the bug happens. However, if you are able to add your field to the PluginDocument when it is being created before it is wrapped in an AnnotatedPluginDocument, then it works.
For example, if you are working with a DefaultSequenceDocument which extends AbstractPluginDocument, you can use AbstractPluginDocument.addDisplayableField followed by AbstractPluginDocument.setFieldValue and then it should work correctly.
We've fixed the bug for the next major release.