Issue with getting post_title

I have a contact form integrated with Mailchimp via Mailchimp for WordPress and the Contact Form 7 integration.

In my CF7 form I am using this code to store the name of given post where the contact form is embedded to a field in Mailchimp:
[hidden mc4wp-BYG "post_title"]

What started happening the last few days, is that instead of sending the actual post title it started saving the word “post_title”.

I thought it was related to the Mailchimp for WordPress plugin, but they pointed me to CF7.

Do you know why there should be an issue with getting the post_title this way all of a sudden?

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