Here is explained how WLS classloading works:
You can try using -verbose:class to JAVA_OPTIONS understand the sequence of events that triggers this ClassNotFoundException, it sounds that the application scope where the class is being used is not the same as where it is included.
1. Do you see any difference when adding the library to user_projects/domains/<MyDomain>/lib?
2. Is there any other version of spring context inside of the application?
3. Do you have defined prefer application packages in weblogic-application.xml?
This MOS document can help also:
WebLogic Server (WLS) Support Pattern: Investigating Different Classloading Issues (Doc ID 1572862.1)
Have you created a package with the package keyword ?
It is a reserved keyword in java...
Are you facing Class not found issues only with the package "
Can you alter the package name for eg xyz.
dummy.NameOfDummyClass and let us know if the issue still occurs ?