libcurl upload - file not appearing

I am trying to upload a file using libcurl (Windows MSVC 2022).  The curl_easy_perform() function call returns CURLE_OK (0).  A verbose trace shows the file bytes uploading and returns "We are completely uploaded and fine".  However, Dropbox subsequently returns a 404 error, and the file is not present on Dropbox.  Any suggestions as to what could be causing this?

The equivalent command line curl command does successfully upload the file.  I tried posting all the details including code and traces, redacting tokens, but my post keeps getting auto-deleted.  I don't know what's getting flagged by the filters.