providing specific watermark to individual user is too difficult. using same username could be the reason.
google search provides...
Websites use "watermarking" (specifically
forensic watermarking) to fight piracy by embedding unique, invisible codes into video streams that identify
who leaked the content, rather than stopping the leak itself. Each copy gets a unique "fingerprint" linked to a specific user or session, so when pirated content appears, rights holders can trace the leak back to the original source and take action, deterring further sharing and identifying bad actors.
How Forensic Watermarking Works
When a user streams or downloads content, an invisible, unique watermark (a pattern of altered pixels or data) is embedded into their specific copy, identifying them as the recipient.
If the content is leaked, pirates might try to strip or alter the watermark, but these forensic marks are designed to survive re-encoding and alteration.
Anti-piracy services monitor the web, find pirated copies, extract the hidden watermark, and use the unique identifier to pinpoint the user or device that originally leaked it.