Upgraded gitlab-ce from 14.10 to 15.0 on Ubuntu 18.04.6 LTS. Afterwards issue notifications stopped being sent on both new issues and issues that had previously been sending notifications. SMTP is not broken though because testing smtp does work:
Notify.test_email('destination_email@address.com', 'Message Subject', 'Message Body').deliver_now
Using that in gitlab-rails sends emails without any problems.
Not sure where to look for errors in the gitlab logs.