I worked for several banks for many years. That included 4 years in a credit card department. Robin would be the responsible party here if he defrauded someone and used their card. In order to prove that, the person would have to press charges and prove fraud to the bank. The chances are, Robin is in conflict with someone who ALLOWED him to use their card - a former friend, ex spouse, etc. And now that person wants their money back. Either way, there are only two options:
1) Robin defrauded that person, and therefore, he’s responsible.
2) That person actually allowed Robin to use their card, and unless they filed a police report and pressed charges due to fraud; THEY are responsible.

You are NOT responsible. Which country are we talking about? I’m in Canada. I suppose the laws are different everywhere, but still.

I strongly suggest you do the following:
1) find a lawyer and pay them for a quick consultation.
2) write to Upwork again saying the way they handled this situation is unethical and possibly not very legal. Demand to see the police report which the cardholder filed. Most likely, there is none and the cardholder simply complained to the bank. The way the bank operated is also unethical. THEY should incur their loss, not you.
3) by the end of the day, Upwork accepted Robin using someone else’s card. THEY are responsible as well. Not you.

Find a lawyer, Google the laws, and start complaining to different organizations: Ombudsman, entrepreneur protection bodies, anyone. The local government if you need to (lol, once my friend wrote to the mayor when someone wronged her). Don’t give up. EVERYONE ELSE is at fault here but you.