Whoa – after more testing, I found the validation error only happens if I try to change the parent page. I can edit other fields on the page and it validates as I expect it to.
I also found the same issue on another site on a WooCommerce product. The validation error happens on a required, but hidden and empty WYSIWYG field, and only when changing the Product Category. I can make changes to other fields and the product validates, even with the hidden, empty, required field.
The fields were WYSIWYG.
I did get this sorted out. In case anyone else has the same issue on a site hosted at WP Engine, I’ll put the fix below.
In WP Engine’s control panel, under SSL, I had to use the following:
Uncheck “Force http:// for non-SSL pages” – if this is checked I was getting a redirect trying to log in.
Select “Use SSL for WP-login” – don’t select the one that includes WP-admin. WP-login only does keep WP-admin https too so I’m not sure what the difference is, but the one that includes WP-admin sets the “Site Address (URL)” to use https://, WP-login only does not.
With these settings, the admin pages are still https, but internal site links in ACF WYSIWYG fields do not get automatically converted to https when resaving. This was happening on 3 sites at WP Engine so I don’t think it was a one off weird configuration or anything.
