Gstreamer you are a thorn in my side.
Whoever decided to have gstreamer’s http support use the HTTP Ranger header for seeking. To the author of that code, it is NOT the server’s job to have YOUR media backend seek, it is YOUR responsibility. Ok it may be reasonable if that part of the media isn’t downloaded but when all of the media is downloaded you are only issuing more HTTP requests to a remote server to get data you already have, why not just use that.
TL;DR Gstreamer it isn’t the server’s responsibility it is yours to take care of media seeking.