Do not know if it was an error somewhere or accepted procedure now.
Recently flew to Cuba from US, did not fill out any forms either going or coming

You flew from a US airport directly to Cuba?

You did not tell the airline in any way, oral or internet or paper, which OFAC category you chose for the general license?

You did not buy a tourist card?

You did not fill out a health form, generally passed out before boarding, or during the flight? (It says "ANEXO 1--Ministry of Public Health of Cuba--Sanitary Statement for Travelers" on the top.)

You did not fill out a customs form (with a big A in the upper left corner, for Aduana, and a blue block in upper right saying "Customs Declaration for Passengers--Welcome to Cuba"), also usually passed out before boarding?

Or are you saying that you imagined you would have to fill out some OFAC application for a specific license?

Sorry, did fill out cards from Cuban govt and paid fee
BUT did not tell any one or put in writing which category I chose, did fly directly from the states, expected that I would be asked by US govt about reason for flight

That's why it's called a "general license". You have only to state that you qualify for the category. And the airline keeps a record in their computer system that would be available to OFAC if needed.
No one has been audited by the government in quite some time. In future, who knows?

just FYI: First time I went, I was asked where I went and what I was bringing in (there was still a quota for cigars and rum at that time), before being told Welcome Home. Second time I went, I was asked, fairly casually, by US immigration at entry, where I had gone and what was my reason for travel, as I passed through the line.