The problem is actually a bit worse than initially reported.
The problem does not only happen after edit/save, but also after restarting the service.
Steps to reproduce
configure correctly (manually add newlines)
Save
Now the openvpn server runs correctly, with the extra-certs option
Go to openvpn status page (/status_openvpn.php#)
Click "restart" on the openvpn service
Result
The service does not restart, openvpn.log shows
Aug 23 17:11:05 fw openvpn[84943]: SIGTERM[hard,] received, process exiting
Aug 23 17:11:06 fw openvpn[1001]: ERROR: Endtag </extra-certs> missing
Aug 23 17:11:06 fw openvpn[1001]: Exiting due to fatal error
To fix again, go the the openvpn config page, manually add the newlines (2x), save -> service runs.