Bye vs Goodbye
When I was reviewing the Greetings skill today, I was asked to translate 'Au revoir'. My answer 'bye' was marked incorrect and the required answer was 'goodbye'. I have always been taught that 'Au revoir' means 'goodbye'.
How would one translate 'bye' to French?
Au revoir is more formal, whereas bye is more casual and informal. I think your answer should have been correct, although here are a few more casual ways of saying goodbye, though I'm not sure if they would be accepted by Duolingo or not: À toute, À plus, À toute a l'heure, À bientôt.
I think it's a bug as I came across that one, too. I reported it and I'm sure others have as well. After all, "bye" and "goodbye" really mean the same thing in English; the latter is more final or formal, but the meaning is identical otherwise.
"Au revoir" is 'bye'. I live in a French-speaking country, so I can confirm this. However, it's more formal as "salut" is more informal.
Duolingo data can are out of its mind at times, submit and hope for the best! Just confirming that you were correct ;-)