To reproduce: - Add a word, press tab the add another word. - Export the document using HtmlFormatProvider. - Import the same document. - The tab is interpreted like 7 spaces.
This is not an issue because there is no tab character in HTML and the tab is replaced with non-breaking spaces. The count of the necessary non-breaking spaces is calculated dynamically so the space in the document looks similar when it is opened in a browser. The side effect here is that there is no way to restore the tab when the document is imported.