This behavior is occuring only if use firefox browser:

1. If you set the EditModes within backend only to EditMode.Preview then all html table elements within content appears like in EditMode.Design with dashed borders.

You can reproduce it on your own demo site https://demos.telerik.com/aspnet-ajax/editor/examples/overview/defaultcs.aspx simple disable EditMode.Html and EditMode.Design.

Now you see the dashed table element borders like in design mode....

2. Sometimes if you switch edit mode from Html to Preview the same affect is occuring with displaying dashed border with html table elements. If you switch directly from Design mode to preview mode all works fine.

3. If you resize the editor in preview mode with resize button on right lower corner the display switch immediatly to dashed borders for all html table elements...

At the moment we use a fix to reset preview mode: If only preview mode is activated for the editor we set the preview mode again with a timeout of 250 ms after loading of dialog is ready (if the timeout is to low no affect appear).
Sys.Application.add_load(function(){ window.setTimeout(function(){$find('AjaxEditorClientID').set_mode(4)},250); }); });

Regards
Thomas