It looks like it is the EWWW Image Optimizer. There are a number of people reporting the issue on the plugin page.
However, switching off the plugin doesn’t clear the issue, which is why I didn’t spot it.
Hi Andy,
Thanks for getting back to me. I am surprised that this is not something that has been requested by more users of the theme. Line 336 of the class-form-generator.php file states:
* The text method creates input elements with type text, and pre-fills them with $_POST values if available.
I was hoping that this would provide me with a hook to achieve my goal in this scenario, but maybe I was mistaken?