I am vary of accepting it for several reasons a) it's pretty informal (cf. with English 'night') b) the Golden Rule might object c) some ppl might complain later that we don't accept dobrej as a shortening of dobrý den and that is something we are never ever going to do.
Lenient or strict, the pattern gets shown regardless and a critical mind can still decern regardless. It remain didactive the same. So keep doing what you're doing. The comments section is there to provide us with extras.
Strictness is how I failed Dutch tests before because of grammar errors in my English lol. But it's still a useful course for non-native speakers. It taught me how to better teach Dutch to foreigners. In fact getting corrected via strictness makes the learners pause and look further.
The issue with accepting or rejecting the nominative is not about what is/not acceptable as a greeting. It has to do with the English side. English sentence fragments in these early skills are accepted without articles. So theoretically someone may just use goofy punctuation for the article-free, non-greeting noun phrase "good night", which would be "dobrá noc" or even "dobrý večer" in Czech.
Until Duolingo grants us our longest running wish, to be able to mark certain translations as marginal, suspicious, or otherwise on the bubble, we have to decide to be overly strict or overly lenient, without the ability to be just right.
The motivation for lengthy debates is usually lessened if the users get a freebie rather than if they feel they were unjustly robbed.