From the looks of it, there are 2 different ways to download the report. One being the download button (green) and the other being a download icon in the viewer (red).
The download button (green) generates the report with the report title given and the .pdf extension as you wanted - while the download icon (red) does the “ReportDownloadServlet”…
I’m not sure why we have two workflows here - I’ll pass this along to the team. Thanks for reporting!