I agree that this is one of those sentences/idioms that you probably won't get the first time. The beauty of Duo though is through repetition one can eventually get an entire section right without losing any hearts! There are no penalties for redoing a section ^_^.
It's not an advantage in this case. I'm a spanish speaker and I couldn't infer the meaning correctly this time, especially because the word "later" doesn't even appear when you sneak peak. I grant you could infer it eventually, but they really should improve this exercise.
Although that might be a correct one-to-one translation, it isn't correct in this case because no one says "Until soon!" as a farewell. A native English speaker might not even know what you're trying to say if you said that. (I am a native English speaker.)
"Until soon, bye" is definitely incorrect, although it is a literal translation. I have never heard this expression in the 25 or so years of being married into an Italian family, so I winged it, but now I know, lol!
I typed "until next time" but was marked down. Surely that is valid and the negative mark just indicates the limitations of the software to handle multiple alternatives? I do like DL by the way so this isn't meant as a damning criticism.
"See you (soon)" is an implied but non-committed intention. It is a way of ending the current meeting without setting a date/time to meet again. "So long" is goodbye... probably with the expectation of meeting again (son0. Idiomatic expressions are not precise.
As usual, I see many of you over thinking & using your own language as a guide instead of your target language. It's often just best to accept that certain things are simply what they are - especially when it comes to idiomatic expressions.