Emails quarantined by the Email Impersonation Control or Data Protection Policy are not visible in the recipient's quarantine portal but the email is visible to the quarantine administrator.
Email Security.cloud
The Data protection option is not enabled in the User Quarantine page.
Dashboard > Platform > User Quarantine Settings
The Data protection option under "Users can view quarantined messages in Email Quarantine interface and summary notifications for the following services" in the page "Dashboard > Platform > User Quarantine Settings" must be enabled.
Make sure the changes you make apply for the affected domain, as each provisioned domain may have custom settings.
Allow up to 1 hour for the propagation before non-admin user can view emails quarantined by the data protection service in the quarantine portal.