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.
