In the assembly application, the work part changes when you create datum by referencing any part file objects in the assembly file. The referenced part file becomes the work part and the datum is created inside the active work part in the assembly file. In the older versions, there was no problem like this. This problem occurs because of a new command in the NX 12.
To solve this problem deactivate the “Allow Automatic Work Part Change” command inside the Top-border bar. If you can not find command in the top border bar, You can use the “Command Finder” to find and deactivate the command. (The command icon is shown in the figure)