CiviMail - 'No recipients': nothing added to table civicrm_mailing_recipients
I tried clearing all kinds of caches and rerunning the Smart Group caching several times but to no avail.

I ended up redoing the entire migration and upgrade, and the problem no longer occurs now.

So it's fixed for me, but I have no idea what setting, add-on or cache was responsible.

EDIT: an issue has been opened for this:
https://issues.civicrm.org/jira/browse/CRM-21856