Is this what you told Monzo when you made the request?
I mean there’s a very high chance that this is some sort of tax evasion tactic. There’s also the risk around the company going out of business and you not getting the service you paid for.
If this is what you told Monzo presumably you’ve fallen into a level of risk that they’re not prepared to accept so they declined your request.
Whether they (and Chase) acted correctly or not is up for debate, and you won’t get an answer from Monzo on why this happened, but I think this is the most likely reason.