Juraj,
What place you mean by "when I go to edit a document of this document type"?
If I go to the CMS Desk, find the document in the content tree, and go to the 'form' tab of that document, the HTML text area field renders as blank. When I click on source, the field is still blank.
Could you please explain where are you checking the HTML in the database?
I have been checking the database using SQL Server mgmt studio. I.e.
SELECT [ArticleText] FROM [custom_Article]
I also confirmed that the documents were previously and are still published.
Would using the API to move the data solve this issue?