It has been reported that the Unsubscriptions report was showing as 0 for the deliveries. However, by checking some of the deliveries, their tracking log journals are showing the unsubscribe events.
The problem is that the “Unsubscribe” links from the email content are recorded as an “Email click” by the tracking log.
For reference, here’s the default (OOTB) Unsubscription link in ACS:
a href="%@valueobject="branding" xpath="@serverUrl" %/lp/bl?pkey=%= escapeUrl(context.profile.PKey)%"_label="Opt-out link" _type="optout"click here/a
By modifying the link type to be “optout”, the Unsubsciptions report will display the metric correctly.