Missing classes from JAR created by sbt assembly

Question

Here's my build.sbt file:

name := "CMDW-Security"

version := "19.11.25"

scalaVersion := "2.11.12"

assemblyJarName in assembly := "CMDW-Security.jar"
test in assembly := {}
fullClasspath in assembly := (fullClasspath in Compile).value
assemblyMergeStrategy in assembly := {
    case PathList("META-INF", xs @ _*) => MergeStrategy.discard
    case x => MergeStrategy.first
}

libraryDependencies ++= Seq(
    "org.scalaj" %% "scalaj-http" % "2.4.2",
    "org.scalatest" %% "scalatest" % "3.0.5" % "test",
    "com.typesafe.play" %% "play" % "2.7.3",
    "org.passay" % "passay" % "1.5.0"
)

I built my JAR using sbt clean compile assembly.

While running my file using spark-submit (I have to run it that way despite not having any spark dependency), I see the following exception thrown:

Exception in thread "main" java.lang.NoSuchMethodError: play.api.libs.json.JsLookup$.apply$extension1(Lplay/api/libs/json/JsLookupResult;Ljava/lang/String;)Lplay/api/libs/json/JsValue;
    at com.ldap.restapi.Authorization.getAuthToken(Authorization.scala:45)
    at com.ldap.executable.Test$.main(Test.scala:21)
    at com.ldap.executable.Test.main(Test.scala)
    at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
    at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:62)
    at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
    at java.lang.reflect.Method.invoke(Method.java:498)
    at org.apache.spark.deploy.JavaMainApplication.start(SparkApplication.scala:52)
    at org.apache.spark.deploy.SparkSubmit$.org$apache$spark$deploy$SparkSubmit$$runMain(SparkSubmit.scala:904)
    at org.apache.spark.deploy.SparkSubmit$.doRunMain$1(SparkSubmit.scala:198)
    at org.apache.spark.deploy.SparkSubmit$.submit(SparkSubmit.scala:228)
    at org.apache.spark.deploy.SparkSubmit$.main(SparkSubmit.scala:137)
    at org.apache.spark.deploy.SparkSubmit.main(SparkSubmit.scala)

When I investigated the JAR by unzipping it using 7zip, I found that play.api.libs.json.JsLookup$.apply$extension1 is indeed not present there. How do I make sure that I get all class files compiled successfully when doing sbt assembly