Appending other uploaded files to a sig request template

I have a template which works just fine with prefill_tags and defined signature placement, etc. The moment I try to add other files i’ve previously uploaded (and captured id’s of) via source_files and additional_documents in the payload, the template becomes null and the signature request will only include those additional documents. That is to say, I cannot find a way to use both a template and add additional files to a signature request. I am using jwt authentication in python.