code block strips fontello font characters from editor with a "?"

Hi,

when I copy and paste <a href="#next-section" title="" class="scroll-down-link" aria-hidden="true" data-av_icon="" data-av_iconfont="entypo-fontello"></a> into the code block the editor changes the data-av_icon="" to data-av_icon="?"

I do have this working on another site on same local setup. As soon as I save/publish the the page and go back into the code block I see the data-av_icon="?"

Things I have tried:
1. copy html code of font/icon from fontello
2 convert back to parent theme
3 It works when I put this code into the Layer slider.

Any help would be appreciated
regards
Michael