Translate plugin does not populate destination if it already exists

[parosio]

Hello,

I think I've found a rather weird behavior of translate plugin (logstash 5.4).

I'm looking for a match in a first dictionary; if no match is found, I need to use a second dictionary.
The second translation only works if I remove the destination field (as in the example below).

Could someone check if this is correct, or if I've overlooked something?

  filter { 
     . . . 
    translate {
    field => "Source_Ip"
    destination => "src_translation"
    exact => true
    fallback => "not_found"
    dictionary_path => "${DICTS}/first_dict.yml"
  }
  if ( [src_translation] =~ /not_found/ ) {
     # it only works removing destination field
    mutate {remove_field => ["src_translation"]}
    translate {
      field => "Source_Ip"
      destination => "src_translation"
      exact => true
      fallback => "not_found"
      dictionary_path => "${DICTS}/second_dict.yml"
    }
  }
 . . . }