we have the problem that emails detected as DLP violation were moved to quarantine according to Cloud Portal.
However, these are not showing up in the customer's quarantine emails for them to release.
How can we solve the problem?
That is correct. You can inform somebody but still the Admin is the only part to release such content.
This is an expected behavior. Just think of DLP as something that is supposed to restrict information from getting in or out of a company and not as an anti-spam solution hence the name "data leakage prevention". The administrator explicitly set these policies as per each respective company's rules. If these DLP quarantined emails can be released by the users then it nullifies what the administrators have set.