The conjunction "а" has both a contrastive meaning and a contradictive meaning. In its contrastive function, it is used to join two or more complete statements that are placed close together for contrasting effect. In this usage it is usually translated as "and", with the connotation "while, whereas".
Have you submitted it as a suggestion? It's only in Beta, so they cannot include every possible translation somebody might come up with (they could never do that, even in a final version). My initial reaction was that "while" is just wrong, but although it's fairly uncommon these days, you're right that it can be used in a contrastive way, similar to Russian "а". So I don't think your translation was wrong - it's just not in the library of possible answers yet - and won't be, unless you report it. "Not accepted" does not necessarily mean "wrong" - especially in a Beta version. They rely on user feedback about other valid answers.
I have reported it. To me it doesn't sound old-fashioned at all. Note that @wizwisdom up here explicitly mentions it as the intended meaning of а.
Mine was a select the right words exercise, and while was among them. So I thought there might be some thought behind not accepting it. (Also I've reported it elsewhere and so far haven't received any email that it has been added.)
(Btw, I would argue that in practice, with the older courses, pretty much all reasonable translations can be and are indeed added.)