API - Post to a linked field

I'm using an API to post to a table and one of the fields is linked to another table. Using postman I have been testing how the data is delivered and I can't get this one field to work :

If I use a Short Text field (id 1344) then the text posts fine. However If I send the same content to a Linked Field (id 1659) then it won't deliver the text, even though it is the same content.

{
"field_1342": "14/10/2019 00:00:00",
"field_1343": "09:38",
"field_1344": "Excel",
"field_1659": "Excel",
"field_1345": "Backup"
}

and the result is

{
"id": "5da435c580325a001546c231",
"field_1348": 1172,
"field_1348_raw": 1172,
"field_1659": "",
"field_1659_raw": [
"E",
"x",
"c",
"e",
"l"
],
"field_1373": "14/10/2019 09:38",
"field_1373_raw": "14/10/2019 09:38",
"field_1349": "Monday",
"field_1349_raw": "Monday",
"field_1342": "14/10/2019",
"field_1342_raw": {
"date": "10/14/2019",
"date_formatted": "14/10/2019",
"hours": "12",
"minutes": "00",
"am_pm": "AM",
"unix_timestamp": 1571011200000,
"iso_timestamp": "2019-10-14T00:00:00.000Z",
"timestamp": "10/14/2019 12:00 am",
"time": 720
},
"field_1343": "09:38",
"field_1343_raw": {
"date": "01/01/2012",
"date_formatted": "01/01/2012",
"hours": "09",
"minutes": "38",
"am_pm": "AM",
"unix_timestamp": 1325410680000,
"iso_timestamp": "2012-01-01T09:38:00.000Z",
"timestamp": "01/01/2012 09:38 am",
"time": 578
},
"field_1344": "Excel",
"field_1344_raw": "Excel",
"field_1345": "Backup",
"field_1345_raw": "Backup"
}

Why has field 1659 delivered so strangely ? How can I resolve this ?