The English translation seems ambiguous to me. One could translate it as "Egy oroszlán sétál a turista mögött," right? That means something different. This English sentence doesn't convey the idea that a lion is walking UP TO THE PLACE behind the tourist. I know this isn't an easy translation, because the Hungarian implies "motion toward" in the form of the word at the end of the sentence. We have to do it by other means, but it's awkward unless you add a bunch of words that aren't (explicitly) in the Hungarian.
Yes, I think I would say something like "A lion walks up behind the tourist" to indicate the transition from not being behind the tourist to being behind the tourist.
In any language ambiguity sometimes occurs. Why not just accept it, if the alternative involves adding ideas that are not in the sentence being translated? In Duolingo we are dealing with isolated sentences, but if they were in a conext the meaning would often be clearer.
But the 'ideas' are there in the Hungarian sentence to be translated; it is the words that would be needed in English that are not there.
In any case, paranoia's suggestion on this page is a good one: 'a lion walks up behind'. This makes it clear that we are not talking about the lion having always been behind the tourist, but rather that it is right now moving behind him.
I suppose you are right that we must sometimes accept ambiguity -- but not without a fight!
English often uses prepositions or helping verbs to express a transition in the state of something (get up, sit down, come over, etc). I think it's entirely appropriate to use those kinds of phrases to translate sentences using words like "mögé," etc.
I always accept/assume that the sentence in the language being learned is correct no matter how awkward it sounds in the native language as long as the exact meaning is captured. In this case the English sentence does not capture the full meaning therefore it should be updated IMHO.
The words are joined when speaking at normal pace, so that the ending consonant of oroszlán gets overshadowed by the first consonant of the next word.