Form Fields Values not merged into generated document

I am experiencing an issue with the Document Generation API when populating Sender Fields created using the Agreement Template Builder.

My workflow:

1. Create a draft envelope using a document template (template has sender fields set).
2. Retrieve the document generation fields using GET /v2.1/accounts/{accountId}/envelopes/{envelopeId}/docGenFormFields.
3. Populate the fields using PUT /v2.1/accounts/{accountId}/envelopes/{envelopeId}/docGenFormFields.
4. Reopen the draft envelope through the DocuSign web interface.

The GET response correctly returns the populated values, for example:

• Client Name: Test Company
• City: Sofia

Additionally, the response reports:

docGenDocumentStatus: mergesucceeded

However, when I proceed to the document preview, the just before preview step shows the sender fields in a box layout with the populated values warning me that those values will be included in the document and then when the document is loaded at the preview stage placeholders remain empty. Downloading the generated document through the API also confirms that the values have not been merged into the document.

1. Confirmation that the put request values are up to date.

   

    

2. Result when clicking on next 

   

   3. Document template with the sender fields where I expect the data to be populated 

      

Can anyone advice what I am doing wrong?