On the new issue view, it's not possible to select Seconds, even if the Advanced settings and Look and feel have the correct settings to show such information.
On the old view, it was possible to edit such information, but due to issues and limitations on the new view, the date settings are not respected anymore.
Show the date time format added to the advanced settings/look and feel.
jira.date.time.picker.java.format - dd/MMM/yy h:mm a s
Look and feel:
Complete Date/Time Format - dd/MMM/yy h:mm a s