Scalatest runner results in VerifyError: Cannot inherit from final class - scala

Question

I am trying to run all the tests (all of them extend FlatSpec) in a jar using the ScalaTest Runner, but get a message for VerifyError. However, I am able to individually run the tests.
The jar was compiled using sbt test:assembly. I am trying to run the tests in another environment where sbt is not available. The tests are available in the jar in com/tfs/test path, as shown in the output below:
samik#samik-lap:~/git/proj$ jar tf test-2018.2.jar | grep MyTest
com/tfs/test/MyTest$$anonfun$1$$anonfun$apply$mcV$sp$1.class
com/tfs/test/MyTest$$anonfun$4.class
com/tfs/test/MyTest$$anonfun$5.class
com/tfs/test/MyTest$$typecreator4$1.class
com/tfs/test/MyTest$$typecreator5$1.class
com/tfs/test/MyTest$$typecreator9$1.class
com/tfs/test/MyTest.class
The following command runs the specific test just fine:
samik#samik-lap:~/git/proj$ scala -J-Xmx2g -cp "scalatest_2.11-3.0.5.jar:scalactic_2.11-3.0.5.jar:test-2018.2.jar" org.scalatest.run com.tfs.test.MyTest
0 [ScalaTest-main] INFO com.tfs.test.MyTest - Starting MyTest test application
Start Time: 0 sec
...
However, when I use Runner to run all the tests (there are ~7 similar tests in the jar available at the same path), it doesn't work.
samik#samik-lap:~/git/proj$ scala -J-Xmx2g -cp "scalatest_2.11-3.0.5.jar:scalactic_2.11-3.0.5.jar" org.scalatest.tools.Runner -o -R test-2018.2.jar
Discovery starting.
*** RUN ABORTED ***
java.lang.VerifyError: Cannot inherit from final class
at java.lang.ClassLoader.defineClass1(Native Method)
at java.lang.ClassLoader.defineClass(ClassLoader.java:763)
at java.security.SecureClassLoader.defineClass(SecureClassLoader.java:142)
at java.net.URLClassLoader.defineClass(URLClassLoader.java:467)
at java.net.URLClassLoader.access$100(URLClassLoader.java:73)
at java.net.URLClassLoader$1.run(URLClassLoader.java:368)
at java.net.URLClassLoader$1.run(URLClassLoader.java:362)
at java.security.AccessController.doPrivileged(Native Method)
at java.net.URLClassLoader.findClass(URLClassLoader.java:361)
at java.lang.ClassLoader.loadClass(ClassLoader.java:424)
How do I get all the tests to run through Runner? How do I get to know what is happening inside and where the issue is? Thanks for any pointer.