Fingerprint unable to fingerprint @timestamp

With input lines like:

{"count":10, "timestamp":"2015-09-22T00:00:00.000Z"}

I am unable to fingerprint based off of the date extracted by my date filter. For fingerprint, it's exactly the same for all of my messages when based off of @timestamp but it is in-fact unique when I use the timestamp field (or count for that matter).

I was able to confirm that the @timestamp field is being created successfully off of the date filter so hopefully I'm just missing something here syntactically?

My config:

filter {
  date {
    match => ['timestamp', 'ISO8601']
# also played with checksum
#  checksum {
#    algorithm => 'md5' # works as expected
#  }
  fingerprint {
    method => 'MD5'
    key => '00000000'
    target => 'fingerprint'
    #source => [ 'timestamp' ] ## works!
    #source => [ 'count' ] ## works!
    source => [ '@timestamp' ] #does not work!

output {
  stdout {
    codec => 'json'
  elasticsearch {
    host => ''
    cluster => 'logstash'
    document_id => '%{fingerprint}'

After posting this I did some more testing and found that the only instance in which fingerprint is not unique is when the timestamps differ in their milliseconds. Fingerprint bug?

You mean between timestamp and @timestamp aren't the same values?

Sorry, no, I mean to say that a timestamp of 2015-09-22T00:00:00.000Z and 2015-09-22T00:00:00.001Z or 2015-09-22T00:00:00.980Z all share the same fingerprint result.