Custom Fields stopped allowing necessary characters

Hi, I’m trying to add more custom fields ( such as “g:title” ) to an already existing feed. It used to be allowed but now more ( and some changes ) are necessary by Pinterest and I’m getting an error saying “Sorry, when creating new custom fields only letters are allowed (so no white spaces, numbers or any other character are allowed).”

As you can see in the screenshot. This feed already exists and those fields have been there for ages but I need a few additional fields and the feed won’t save these new custom fields but still runs old version of it with no issues.

https://ibb.co/cXRWKRr

Thanks in advance for the help and your plugin!!