This is the full original input json.txt (i used jq for pretty output):
{"id":1,"timestamp":"2019-09-12T13:43:42Z","paymentType":"Amex","name":"Merrill Duffield","gender":"Female","ip_address":"132.150.218.21","purpose":"Toys","country":"United Arab Emirates","age":33}
{"id":2,"timestamp":"2019-08-11T17:55:56Z","paymentType":"Visa","name":"Darby Dacks","gender":"Female","ip_address":"77.72.239.47","purpose":"Shoes","country":"Poland","age":55}
{"id":3,"timestamp":"2019-07-14T04:48:25Z","paymentType":"Visa","name":"Harri Cayette","gender":"Female","ip_address":"227.6.210.146","purpose":"Sports","country":"Canada","age":27}
{"id":4,"timestamp":"2020-02-29T12:41:59Z","paymentType":"Mastercard","name":"Regan Stockman","gender":"Male","ip_address":"139.224.15.154","purpose":"Home","country":"Indonesia","age":34}
{"id":5,"timestamp":"2019-08-03T19:37:51Z","paymentType":"Mastercard","name":"Wilhelmina Polle","gender":"Female","ip_address":"252.254.68.68","purpose":"Health","country":"Ukraine","age":51}
This is the full output (out.log):
{"log":{"file":{"path":"/tmp/json.txt"}},"@version":"1","timestamp":"2019-08-11T17:55:56Z","id":2,"gender":"Female","@timestamp":"2022-07-08T14:15:53.878804Z","event":{"original":"{\"id\":2,\"timestamp\":\"2019-08-11T17:55:56Z\",\"paymentType\":\"Visa\",\"name\":\"Darby Dacks\",\"gender\":\"Female\",\"ip_address\":\"77.72.239.47\",\"purpose\":\"Shoes\",\"country\":\"Poland\",\"age\":55}"},"paymentType":"Visa","host":{"name":"NBK-DAVIDD"},"type":"json","ip_address":"77.72.239.47","age":55,"purpose":"Shoes","message":"{\"id\":2,\"timestamp\":\"2019-08-11T17:55:56Z\",\"paymentType\":\"Visa\",\"name\":\"Darby Dacks\",\"gender\":\"Female\",\"ip_address\":\"77.72.239.47\",\"purpose\":\"Shoes\",\"country\":\"Poland\",\"age\":55}","name":"Darby Dacks","country":"Poland"}
{"log":{"file":{"path":"/tmp/json.txt"}},"@version":"1","timestamp":"2020-02-29T12:41:59Z","id":4,"gender":"Male","@timestamp":"2022-07-08T14:15:53.880454Z","event":{"original":"{\"id\":4,\"timestamp\":\"2020-02-29T12:41:59Z\",\"paymentType\":\"Mastercard\",\"name\":\"Regan Stockman\",\"gender\":\"Male\",\"ip_address\":\"139.224.15.154\",\"purpose\":\"Home\",\"country\":\"Indonesia\",\"age\":34}"},"paymentType":"Mastercard","host":{"name":"NBK-DAVIDD"},"type":"json","ip_address":"139.224.15.154","age":34,"purpose":"Home","message":"{\"id\":4,\"timestamp\":\"2020-02-29T12:41:59Z\",\"paymentType\":\"Mastercard\",\"name\":\"Regan Stockman\",\"gender\":\"Male\",\"ip_address\":\"139.224.15.154\",\"purpose\":\"Home\",\"country\":\"Indonesia\",\"age\":34}","name":"Regan Stockman","country":"Indonesia"}
{"log":{"file":{"path":"/tmp/json.txt"}},"@version":"1","timestamp":"2019-09-12T13:43:42Z","id":1,"gender":"Female","@timestamp":"2022-07-08T14:15:53.852279Z","event":{"original":"{\"id\":1,\"timestamp\":\"2019-09-12T13:43:42Z\",\"paymentType\":\"Amex\",\"name\":\"Merrill Duffield\",\"gender\":\"Female\",\"ip_address\":\"132.150.218.21\",\"purpose\":\"Toys\",\"country\":\"United Arab Emirates\",\"age\":33}"},"paymentType":"Amex","host":{"name":"NBK-DAVIDD"},"type":"json","ip_address":"132.150.218.21","age":33,"purpose":"Toys","message":"{\"id\":1,\"timestamp\":\"2019-09-12T13:43:42Z\",\"paymentType\":\"Amex\",\"name\":\"Merrill Duffield\",\"gender\":\"Female\",\"ip_address\":\"132.150.218.21\",\"purpose\":\"Toys\",\"country\":\"United Arab Emirates\",\"age\":33}","name":"Merrill Duffield","country":"United Arab Emirates"}
{"log":{"file":{"path":"/tmp/json.txt"}},"@version":"1","timestamp":"2019-08-03T19:37:51Z","id":5,"gender":"Female","@timestamp":"2022-07-08T14:15:53.880880Z","event":{"original":"{\"id\":5,\"timestamp\":\"2019-08-03T19:37:51Z\",\"paymentType\":\"Mastercard\",\"name\":\"Wilhelmina Polle\",\"gender\":\"Female\",\"ip_address\":\"252.254.68.68\",\"purpose\":\"Health\",\"country\":\"Ukraine\",\"age\":51}"},"paymentType":"Mastercard","host":{"name":"NBK-DAVIDD"},"type":"json","ip_address":"252.254.68.68","age":51,"purpose":"Health","message":"{\"id\":5,\"timestamp\":\"2019-08-03T19:37:51Z\",\"paymentType\":\"Mastercard\",\"name\":\"Wilhelmina Polle\",\"gender\":\"Female\",\"ip_address\":\"252.254.68.68\",\"purpose\":\"Health\",\"country\":\"Ukraine\",\"age\":51}","name":"Wilhelmina Polle","country":"Ukraine"}
{"log":{"file":{"path":"/tmp/json.txt"}},"@version":"1","timestamp":"2019-07-14T04:48:25Z","id":3,"gender":"Female","@timestamp":"2022-07-08T14:15:53.879848Z","event":{"original":"{\"id\":3,\"timestamp\":\"2019-07-14T04:48:25Z\",\"paymentType\":\"Visa\",\"name\":\"Harri Cayette\",\"gender\":\"Female\",\"ip_address\":\"227.6.210.146\",\"purpose\":\"Sports\",\"country\":\"Canada\",\"age\":27}"},"paymentType":"Visa","host":{"name":"NBK-DAVIDD"},"type":"json","ip_address":"227.6.210.146","age":27,"purpose":"Sports","message":"{\"id\":3,\"timestamp\":\"2019-07-14T04:48:25Z\",\"paymentType\":\"Visa\",\"name\":\"Harri Cayette\",\"gender\":\"Female\",\"ip_address\":\"227.6.210.146\",\"purpose\":\"Sports\",\"country\":\"Canada\",\"age\":27}","name":"Harri Cayette","country":"Canada"}
How you can see there is no "test" field in the output file.