Thanks for the report. I really don’t have the time to fix this but anyone who wants to give it a shot can fork the plugin at github and send a pull request with a fix:
https://github.com/bonny/WordPress-Simple-Fields
I have been trying to come up with a solution to this issue for my sites and I have narrowed the issue down to the visual editor. If you do not enable the visual editor for textarea fields, the issues goes away. The problem appears to be related to WordPress changing the way that the tinyMCE editor is loaded. It is now loaded using the Rest API and it is set so that it only loads when the editor is opened initially thus causing the Ajax issue. I am sure there is a change that can be made to work around this update but I am beyond my depth with it. Does anyone else have an idea how to resolve this issue?
I would love to find a solution for this also
