Still think "is asleep" should not be accepted because that's an adjective and wouldn't use verb form in HV, but anyway...
When it comes to translation, only the meaning is important. The words and structures may be completely different. In English, "is sleeping again" and "is asleep again" have the exact same meaning. So they can be translated from the same HV sentence.
I will add that one should always translate as literally as possible on Duolingo though, to avoid running into issues like being marked incorrect( unless that is specifically something you are trying to help with ).
We use complex tenses in English that other languages just don't use, although they may support them. I believe that HV can actually be structured like the English sentence as well but since I am not very good at modifying the words yet, I will use another language as an example.
In Esperanto, we would say "Gxon denove dormas", which uses a structure like [noun]-[adverb]-[verb]. However, we can also use the English-like structure if truly desired. "Gxon denove estas dormanta" so now it's structure is [noun]-[adverb]-[verb]-[adjective] where the adjective comes from turning the verb root into the active participle in the present tense.
Well, "to be asleep" is an English expression, and there is nothing that says HV has to express it in the same way i.e. with an adjective. In fact, I think it's likely that it does not.