the other one should also be accepted. reported

I said "and the other is tall" because on a different sentence I was counted wrong for not including "and" even though the construction of the sentence was the same.....a comma followed by "a másik". This time I was counted wrong for including "and". It seems inconsistent. Neither time used "és".

The English sentences have to get more lenient, it seems no matter what I type it's always wrong and I end up doing this one over and over again.

"The one girl is short, the other girl is tall" was marked incorrect

"One girl is short, the other is tall." Yes?

By my opinion your translation is not correct, since "One of the girls is short, the other one is tall." means "Az egyike a lányoknak alacsony, a másik magas.". "Az egyik lány alacsony, a másik magas." should be translated to "The one girl is short, the other one is tall.". Your translation uses girls (plural), and missing the definitive article (az).

Why not: "The one girl is tall, the other short"?

If we're going to skip the conjunction, why not skip the repetition of "is"?