Can't assign a lead to a user via API on creation

Hi all,

I'm looking for a little help with the SugarCRM API.

I'm trying to create a lead, using the API, and would like to assign that lead to a specific user.

This is the data I'm currently passing:

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first_name: "TEST",
last_name: "TEST",
email: [
    {
        "email_address":"test@test.com",
        "primary_address":"1",
        "invalid_email":"0",
        "opt_out":"0"
    }
],
assigned_user_id: 'some active user id'
XXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXX

first_name: "TEST",
last_name: "TEST",
email: [
    {
        "email_address":"test@test.com",
        "primary_address":"1",
        "invalid_email":"0",
        "opt_out":"0"
    }
],
assigned_user_id: 'some active user id'

This is the endpoint I'm hitting:

/rest/v11_8/Leads

From what I've read elsewhere on the forum, adding the assigned_user_id field should be sufficient to assign the lead to the specified user, however it seems to randomly assign them to another user.

Is there anything else I need to check?

Any advice appreciated.

Thanks