add_field doesn't appear to work using date filter

[krisATelastic]

• Version: 6.7.x
• Operating System: Ubuntu (DEB Package)
• Config File (if you have sensitive info, please remove it):

input { stdin { codec => json } }

filter {
  date {
    add_field => { "logtimestamp" => "%{[date]} %{[time]}" }
    match => [ "logtimestamp", "dd/MM/yy HH:mm:ss.SSSSSS" ]
  }
}

output { stdout {} }

• Sample Data: {"date":"19/04/25","time":"10:20:34.443879"}
• Steps to Reproduce:
  Using the above configuration and data, you'll see that the add_field does not construct the logtimestamp field to simplify building a field to match the datestamp using a single filter call.

{
      "@version" => "1",
          "time" => "10:20:34.443879",
          "host" => "lstest",
          "date" => "19/04/25",
    "@timestamp" => 2019-05-13T11:54:57.645Z
}

The same add_field configuration works when using mutate instead no problems, but figured i'd report this for consistency.

input { stdin { codec => json } }

filter {
  mutate {
    add_field => { "logtimestamp" => "%{[date]} %{[time]}" }
  }
  date {
    match => [ "logtimestamp", "dd/MM/yy HH:mm:ss.SSSSSS" ]
  }
}

output { stdout {} }

Output

{
      "@timestamp" => 2025-04-19T00:20:34.443Z,
            "host" => "lstest",
            "date" => "19/04/25",
            "time" => "10:20:34.443879",
        "@version" => "1",
    "logtimestamp" => "19/04/25 10:20:34.443879"
}

[TheVastyDeep]

That is expected. add_field is one of the common options ("decoration") that is performed when a filter has successfully executed. Also, the date filter is a no-op if the field to be matched does not exist. So the filter does not successfully match the field, so it does not add_field you are asking it to match.