LDAP-based group has been created with "Enable Personal Quarantine Automatically" option enabled under the Activation tab.

However, when you search for a single user from the Users page, this user does not have the Personal Quarantine enabled.

Why this user has not inherited the Group's global setting? Is there a way to mass-enable the PQ for all users that belong to a particular group?

When "Enable Personal Quarantine Automatically" option is enabled in the Group settings, this means that the PQ access wills be enabled automatically when a user from the group has received an email.

If a single user shows with PQ not enabled, it is because this option has not yet been activated. When you send an email to this account and reopen his user preferences, it will show with PQ enabled.

Although it is not needed to force enabling the PQ for any user, the easiest way to do it is to send an email to each and every user from the group in question.