Welcome to OFAC compliance. In case you did not know, it is the finance sector's job to make sure you do not transact with any embargoed or sanctioned entities or groups the United States decides shouldn't be given entrance to the U.S. financial system. This responsibility is part of doing business.
You will not find a licensed payment processor that will not do this type of surveillance/due diligence, as they are strictly liable for any violations. If this creeps you out, join the club.
You refer to your transaction as private and yet you use Venmo… I don’t think “private” means what you think it means.
You need to educate yourself about FinCEN (https://en.wikipedia.org/wiki/Financial_Crimes_Enforcement_N...). You'll end up with the same issue with alternatives, as no business which handles payments is immune to having to abide by the FinCEN rules.
And because Cuba is likely still a country listed as sanctioned, the mention of "Cuban" in the description likely triggered a higher level check to verify you were not trying to transfer money to Cuba via Venmo (which would run afoul of the sanctions).