When editing a zone only one NS record can be added. When adding additional NS records, the confirmation message indicates more and additional NS record was added but it does not show up in the list of NS records.
This should be fixed in coming update.
This problem persists in version 2.0 build 3. Also doesn’t work when using the Cyberpanel CLI.
As a workaround you can use pdnsutil add-record on the command line.