Iteration and loop in logstash

Elastic Stack Logstash
3

I've tried the split plugin, but I found a bizarre behavior...

filter {
  split {
    field => "Attribute"
    target => "book_bundle"
    periodic_flush => "false"
  }
}

Which will take my source event, clone it into N new events, where N is the number of members of the Attribute array. The array member will be put into the book_bundle field.
and then I added mutate with gsub option to transform book_bundle field and place it into a book1 .. bookN fields...

The problem here, when my logstash process is running, I find only the first element transformed like I want.. and then when I stop the process I find the rest of array's elements in the output file!

I can't understand this behavior.