Python Requests POST timeout prematurely closed connection

Question

I'm doing some file uploads that sends to an nginx reverse proxy. If I set the python requests timeout to 10 seconds and upload a large file, nginx will report client prematurely closed connection and forward an empty body to the server. If I remove the requests timeout, the file uploads without any issues. As I understand it, the timeout should only apply if the client fails to receive or send any bytes, which I don't believe is the case as it's in the middle of uploading the file. It seems to behave more like a time limit, cutting the connection after 10 seconds with no exception being raised by requests. Is sending bytes different than reading bytes for timeout? I haven't set anything for stream or tried any type of multi-part. I would like to set a timeout but confused as to why the connection is getting aborted early - thanks for any help.