The frontend allow the rendering of fields thanks to converters and their value_to_html and record_to_html methods. These methods were changed with https://github.com/odoo/odoo/commit/3407fb70e4a5b16174234d10d97710572b6f808c#diff-d5a9a5d91bd3171e30e7facd628bdf5cL411. The problem is that the web_editor also contains code which relies on these methods and it was not adapted.