"¡Adiós, hasta mañana!"
Translation:Bye, see you tomorrow!
They do indeed! One because of it's more frequent/modern use and the other because of the origin of the greeting. It's like how in Spanish and French etc. where we say adios and adiu; in English we first said "God be with ye" then it became "God be (with) ye" and then "god-b-ye" and then when more people became literate they assumed it was "good-bye" and knowledge of the origin became lost to most people.
"goodbye until tomorrow" should also be accepted as it is on translation sites and in common interpretation, but when I went to report it as "my answer should also be accepted" that is no longer a choice--the only choices were "The audio does not sound correct. The Spanish sentence is unnatural or has an error. The "Correct solution" is unnatural or has an error." and none of those really fit....wish they would allow you to report and explain....
Generally Duo won't give credit for typos if the typo results in a legitimate word in the target language. So, for example, if you had to type the English equivalent of a sentence (so English is the target language), while it might accept "tje" instead of "the", it probably won't accept "he" because "he" is a legitimate word in English (but the wrong word when you're after "the").
I put down the right word, but used the wrong...what do you call that word? I put down "by" instead of "bye"
Some are. It allows you to not put the leading ¿ on questions, for instance.
As for spelling, if the error creates a word that is not an actual word in the target language, it may allow it if it can work out what you were aiming for. Also remember that the accent marks can be very important - for example: "papá" means "dad", but "papa" means "potato"; and "tú" means "you", but "tu" means "your". And, ñ is as much a different letter from n, as m is from n.