I think I’ve had something similar happen before:
The replies to whisper posts are automatically created as whispers, so if post numbers change whilst a reply is being created then you can occasionally (though very rarely) get something accidentally created as a whisper by a ‘non-whisper’ user.
To tidy up and convert the whisper to a regular post you can install the Toggle Whisper 👁 theme component (or use the API directly, depending on what you’re comfortable with)