I have a frontend submission textarea field for inserting a shortcode from a third plugin.
When I do it the first time it works fine,
when I open the frontend form to edit anything, this field appears formatted as html and stops working.
It's so weird. The field itself does not support parsing the shortcode to the HTML code like that. Please try to deactivate all plugins except Meta Box, MB extensions, switch to the standard theme of WordPress then re-check this issue.