"learned afterwards" is correct and can be used in written English.
You can use it when referring to something you learned after a certain period of time. For example, "I learned afterwards that he had been released from jail."
He only learned afterwards what he was expected to do.
I learned afterwards that Springsteen and Scialfa's silver wedding anniversary fell just three days after the Wembley show.
He was a tourist called Basilio, I learned afterwards, from Buenos Aires, where he had performed in a Yiddish theatre troupe.
It's very frustrating, as a minor participant, to have learned afterwards that that the head of MI6 felt the intelligence was being made to fit around the policy.
I learned afterwards that she had known herself to be mortally ill for nearly a year, and had resolutely avoided seeing anyone she knew".
I learned afterwards that my friend, who spoke some Bosnian, had told the host, 'I don't feel well, but my friend here is starving'.
I learnt afterwards that the first two men behind me had been wounded and the third killed … Presently I saw two men moving cautiously below me.
Escaping en route, she made it back to Prague by late March, and learned soon afterwards that her beloved Sasha had died in Kraslice camp.
Clark listed the visit in the itinerary he submitted to the ambassador, but he learned only afterwards that it was not approved.
The family learned soon afterwards that the 9-year-old's teacher, who was running the marathon, was safe as well.
Immediately after practice, the "motor memory" of A is fragile and retrograde interference may occur if another task, B, is learned shortly afterwards [2], [4].
