Reported upstream: https://secure.phabricator.com/T6215
Private tasks are reflected in the timeline, just not showing their subjects/links to those without the permissions to access them (very good!), i.e.
Qgil closed blocking task Restricted Task as "Invalid".
However, these private tasks are not listed at the Blocks / Blocked By fields of the task. Whoever can access them do see them, for the rest they don't exist. It would be better if they would also appear, just as "Restricted task".