When you have custom configuration groups, click the HDFS service's configs, it shows you the default config group. Expand custom hdfs-site.xml, it shows you a option of "add property".
Choose your custom config group, the "add property" button disappear forever. It won't come back when you switch back to the default group.
Reload the page to get the "add property" button back.
This affects all the services with the Custom xxx-site.xml configs.