I upgraded to 5.19.0 and noticed that every report for completed mailings listed on the Find Mailings screen is now showing this message in the Delivery Summary section:
Delivery has not yet begun for this mailing. If the scheduled delivery date and time is past, ask the system administrator or technical support contact for your site to verify that the automated mailer task ('cron job') is running - and how frequently.
Despite this, the reports still show the Click-through Summary and other sections.
I see this issue on both my production and development systems. I tried clearing cache. Other CiviMail reports (from the Reports tab) seem to be correct.