Service Request creation - Sold-to party: System Validation cannot find BP

Hi. I am fairly new to BSP and CRM, but I have this problem:

I have a Z validation on GET_SOLDTO_NAME and it seems to work ok, but it looks like after passing that, the standard system checks again and it changes the value to that text, but uppercase and adding the text "(does not exist)", e.g. Original: 'Mr. Freddy'. Changed: 'MR. FREDDY (does not exist)'. However, the BP actually exists and can be visualized in the tx. PPOMA_CRM

I do not know what message number is it, but the text reads:

Diagnosis

You have entered a name or number that does not exist in the business partner master data. This can be for the following reasons:

The person, organization, or hierarchy has not been defined in the master data.

The name or number represents a partner who has been defined in an area other than business partner master data, for example, in Human Resources.

System Response

The system cannot determine the business partner.

For example, the system cannot recognize an employee's user ID as a business partner unless the employee has been entered as a business partner, and the master data includes his or her user ID.

How can I find where this check is made and why does it change the value?