Character properties applied on the last paragraph symbol for a paragraph in a list, e.g. font size, are not exported to HTML when StyleExportMode is Inline.
As a side effect, when the document is imported back, and the text of the bullet is deleted, its properties switch back to the default ones.
Workaround: Change the styles to export as inline properties:
htmlProvider.ExportSettings.StylesExportMode = StylesExportMode.Classes;
Please be aware that this may cause side effects, as described in this issue: RichTextBox: Character properties of the paragraph symbol are exported to HTML for the whole list item when StyleExportMode is Classes