Just started to experience this recently on our Betheme WP site. I add a column text block and when I paste into the editor on the VISUAL site the widget toggles to the EDIT side and places the similar html.

<span data-mce-type="bookmark" style="display: inline-block; width: 0px; overflow: hidden; line-height: 0;" class="mce_SELRES_start"></span>

If I then toggle back to VISUAL - the editor disappears- when then toggling to EDIT it duplicates the code again and again and again as I toggle back and forth. Page refresh fixes but then happens again with new blocks.

Having a developer look at it now.