Reference not working

We want to generate a PDF with a unique ID in the file name. In the general settings I have set Stagevacature-[reference].pdf is file name, but this is not working. The name of the PDF attachment I receive is without the reference code filled in (so the name is still Stagevacature-[reference].pdf). In the upload directory the file name is Stagevacature-[reference]-5f5f515cc63bd.pdf and the identifier is not unique (every PDF gets the same reference number). Can you please let me know why the reference is not working?
Thanks in advance!

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