Part of the job of the platform provider is to make sure that both parties are genuine. That the buyer is who they say they are and is not using stolen money, a

Part of the job of the platform provider is to make sure that both parties are genuine. That the buyer is who they say they are and is not using stolen money, and that the seller (in this case you) actually delivered what has been contracted. Upwork has failed at job 1, and one presumes continues to fail at it unless they have introduced all-new sign-up/diligence processes since then.