How to keep a Workpackage visible on the task board, also when all cards are closed?

The default behavior of Lynx is that when all cards are closed, the workpackage's status is set to "completed". This means it will no longer show in the workpackages list. 

However, if you wish to manually set the workpackages to complete instead, you can change this in the configurations via:

Configure > Space Properties > Turn features on/off tab > Uncheck "Auto complete workpackages."