Empty cell is getting created in multi select control when spell check is enabled.   Please check attached attachment for detailed steps to reproduce and issue screen shot. 

steps to reproduce :

1. Create simple flow with single Flow action and Section
2. Include Multiselect control and Rich Text editor( RTE)
3. Configure multiselect control. Ex: display all case attachments
4. Enable spell check for RTE
5. Created non-auto generated section
6. Include below script in non-auto generated section

          <script type="text/javascript">

$(document).ready(function() {

  CKEDITOR.on( 'instanceReady', function( evt ) {

    var editor = evt.editor;

    editor.document.$.body.setAttribute('spellcheck',true);

  });

});

 </script>

7. Include non-auto generated section in main section, referred in Flow Action.
8. Run the flow and at run time in RTE enter some text.  Wrong spelling words will be highlighted with red color line.
9. Mouse over on red highlighted lines. System gives spell check suggestions. 
10. Observe multiselect control empty box is created in multi select.   This is correct User experience. 
11. This behavior seen only when multi select control is present in section.

followed instructions in below link  still exist. 

Auto Spell Check Feature | Support Center (pega.com) 

issue screen shot: