Mailing Subscription form does not validate reCaptcha

To reproduce:

• Go to: https://civicrm.org/civicrm/mailing/subscribe?reset=1 (or setup any site with a reCaptcha key, it automatically enables on this form).
• Enter an email, select a newsletter
• Hit Submit, without answering the Captcha.

Result: captcha is not validated.

I tried debugging to the point where in CRM_Core_Utils_ReCAPTCHA::add(), the call to isSubmitted returns false, but that may have been a misleading, since removing this bit seems to fix it:

https://lab.civicrm.org/dev/core/-/blob/master/CRM/Mailing/Form/Subscribe.php#L123

      // If this is POST request and came from a block,
      // lets add recaptcha only if already present.
      // Gross hack for now.
      if (!empty($_POST) &&
        !array_key_exists('recaptcha_challenge_field', $_POST)
      ) {
        $addCaptcha = FALSE;
      }

I don't know what "came from a block" means in this context. Mayeb an old Drupal block?

It was added in 2009: https://github.com/civicrm/civicrm-svn/commit/e745abc5114e644ef58712a42e73af9be100588c#diff-7e49ca39c25024fb4dbba97bee79be03

Related: infra/ops#946 (closed)