This is not a bug, it is due to the ConvertFontToSpan
. When switching to HTML mode, font elements are replaced with span ones. In Design - span to fonts. The RadEditorHtmlInspector shows the content of the Design and it is not designed to correspond to the HTML mode.
You can just disabled the ConvertFontToSpan
content filter to prevent the conversion. You can see how in the Content Filters article
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