Window+collect_list causing single-task operation

Hi,

I had this problem in "real" environments and also made a self-contained test ( [^Test.scala] attached).

Having this Window definition:

val myWindow = Window.partitionBy($"word").orderBy("word") 
val filt2 = filtrador.withColumn("avg_Time", collect_list($"number").over(myWindow))

In the test I can see how all elements of my DF are being collected in a single task.

Unbounded+unordered Window + collect_list seems to be collecting ALL the dataframe in a single executor/task.

groupBy + collect_list seems to do it as expect (collect_list for each group independently).
 
At the end it was a data distribution problem caused by one table which was supossed to do 1-1 join and did 1-N join (!!)