In Nexus, when you use the Postscript RIP, the default behavior allows you to release single separations in the workflow. When using the PDF RIP, the default behavior is a grouped output. There are settings that allow to achieve similar behavior as before.
Open your RIP workflow and double click the Rasterizer.
In the newly opened window go to More options.
At the bottom of the new parameters window you will find Group Separations enabled. Disable this setting and enable RIP Single Separation option.
Confirm by clicking OK.
Confirm again in the RIP parameters window.
Run a test in the workflow, you will see that for every separation a new line is created in the job, allowing to release only one separation when needed.