Spark closures behavior in local mode in IDEs - scala

Question

I am observing weird behavior of spark and closures in local mode on my machine v/s a 3 node cluster (Spark 2.4.5).
Following is the piece of code
object Example {
val num=5
def myfunc={
sc.parallelize(1 to 4).map(_+num).foreach(println)
}
}
I expected this to fail regardless since the local variable num is needed in the closure and therefore Example object would need to be serialized but it cannot be since it does not extend Serializable interface.
when I run the same piece of code from spark-shell on my same local machine, it fails with the error given the rationale above:
When I run the same piece of code in yarn mode on a 3 node EMR cluster, it fails with the exact same error as in the above screenshot...given the same rationale as mentioned above.
when I run the same piece of code in local mode on a the same cluster (=> master node), it also fails. The same rationale still holds true.
However, this, when I run from an sbt project (not a Spark installation or anything...just added Spark libraries to my sbt project and used a conf.master(local[..]) in local mode runs fine and gives me an o/p of 6,7,8,9:
This means its running fine everywhere except when you run it by adding Spark dependencies in your sbt project. The question is what explains the different local mode behavior when running your Spark code by simply adding your Spark libraries in sbt project?