I wanted to test a new module, so installed a new MagicMirror. After restarting it did nothing. Autostart did not work.
Not on 2019-04-08-raspbian-stretch or 2019-09-26-raspbian-buster
This is what I am doing now to get autostart working again.
After restart give the following commands:
pm2 unstartup systemd
sudo env PATH=$PATH:/usr/bin /usr/lib/node_modules/pm2/bin/pm2 startup systemd -u pi --hp /home/pi
pm2 start MagicMirror/installers/pm2_MagicMirror.json
Works for me.