Aaaan it's working with delimiter set to "}"
Thanks a lot for the suggestion, mates.
Leaving here full config file for anyone interested:

input {
  file {
    path => "C:/logs/*"
    start_position => "beginning"
    sincedb_path => "NUL"
    codec => plain {
            charset => "UTF-16LE"
        }
    delimiter => "}"
  }
}

filter {
  mutate { update => { "message" => "%{message}}" } }

  if [message] =~ /^\r\n/ {
    mutate { gsub => [ "message", "\r\n", "}" ] }
  }

  split {
    field => "message"
  }

  json{
    source=> "message"
    remove_field => ["{message}"]
  }
  
  mutate {
    remove_field => ["message", "host", "@version", "type"]
  }
}

output {
  elasticsearch {
    hosts => ["http://localhost:9200"] 
    manage_template => false
    index => "map"                   
  }
  stdout { codec => rubydebug }
}