That's what happening to me exactly. I was trying to post full season of Chernobyl yesterday at filejoker.net and it would start at 8-10 MB/s and within minutes fell down to like 400-800 KB/s.It was throttled on some file hosting sites.
Sometimes the upload would start at 75 Mbps, but within a few minutes it would drop down to around 2-3 MB/s.
Complaining will not help because they will just ask you to run a speedtest and that shows 75 Mbps upload, so they will close the complaint.
I will not just complain, i will cancel my connection if they don't fix this. I am not paying 12,000 Rs monthly to stream youtube.