What is the hotfix you are using and have you checked the bug-tracker yet? I am asking because in hotfix no.93 was fixed a bug:
WYSIWYG editor - HTML sanitizer removed 'mailto' links
The HTML sanitizer in the Rich text editor component for the page and form builder removed 'href' attributes containing 'mailto' links from < a > tags. The issue occurred after applying hotfix 13.0.89 or newer.
If you are using hotfix 89-92, then the issue might be caused by the bug. If you are using newer hotfix, please provide clear and detailed steps how to reproduce the issue. I just checked the 13.0.124 version using Dancing Goat and the out of the box rich text widget and everything seems to be working fine.