We contacted Apple Developer Technical Support (DTS) with this issue.
They replied that this is a bug in Xcode 8.
We submitted a bug report and hope for a quick update. (Apple Bug Report ID: 28315920).
If you experience a similar issue, please also file a bug report (referring to ours) so the Apple engineers see its not a single case.
Update for Xcode ≥ 8.1
The problem seems fixed now, at least for the delegate methods we are using in our project.