Netsuite external Id error depending on externalid value

I'm simply trying to upsert a contact in Netsuite using REST api, via Postman.

This contact is not present in NS, none of the tested external ids is.

I send this request

PUT {{REST_SERVICES}}/record/v1/contact/eid:a0A580000010LVvEAM
with body {
	"customForm":"107",
	"email":"user@example.com",
	"externalId":"a0A580000010LVvEAM",
	"firstName":"first",
	"lastName":"Name",
	"salutation":"Mrs.",
	"subsidiary":2
}

The server returns this message :

{
    "type": "https://www.w3.org/Protocols/rfc2616/rfc2616-sec10.html#sec10.4.5",
    "title": "Not Found",
    "status": 404,
    "o:errorDetails": [
        {
            "detail": "The record instance does not exist. Provide a valid record instance ID.",
            "o:errorCode": "NONEXISTENT_ID"
        }
    ]
}

which is already weird since an upsert, by definition, doesn't need an internal id nor an existent external id.

Now I tried to remove the last character from the extId, just to see what would happen :

PUT {{REST_SERVICES}}/record/v1/contact/eid:a0A580000010LVvEA
with body {
    "customForm":"107",
    "email":"user@example.com",
    "externalId":"a0A580000010LVvEA",
    "firstName":"first",
    "lastName":"Name",
    "salutation":"Mrs.",
    "subsidiary":2
}

and I get my 204 No Content success return, the contact is inserted.

I then tried with a 'N' as 18th character instead of the 'M', it worked too.

I'm quite confused about this issue, why an external id would work and another one wouldn't ?

To be honest, everything you have shared sounds completely strange; although not surprising given that I have also seen strange behaviours of the NetSuite system. From my perspective, everything mentioned sounds like a defect, but here are some areas I would comment on:

1. If you are querying the external ID in the URL, is it actually necessary to include it as a parameter in your body? Keep things as simple as possible to reduce the risk of these problems.

2. Have you tried running a saved search based on the external ID you have queried? It's worth just checking that the error messages returned are actually correct (i.e. "The record instance does not exist."). Indeed, given the strange behaviours seen, I think we have reasonable grounds to lose confidence in NetSuite's web services.

3. Do you get the same responses when you are using the "administrator" roles as part of the authentication? It's worth just ruling out roles/permissions as a possible factor for the behaviour.

Lastly, I would add that I have always avoided using the PUT call and instead stick to POST and PATCH. PUT has let me down many times in the past.

Although not a conclusive answer, I hope the above might be of some help to you.