The accepted translation is "Ninjas work at night" which is a general sentence and inserting a definite article would change the meaning (and sounds unnatural to my non native ear). Is the definite form "natten" necessary in Danish? Would indefinite form mean something different?
I think (non-native) that in Danish you always put the definite form. It is, in fact, rather strange how it is done in English (switching between definite and indefinite from time to time):
"in the morning" -> "om fromiddagen"
"at noon" -> "om middagen"
"in the afternoon" -> "om eftermiddagen"
"in the evening" -> "om aften"
"at night" -> "om natten"
Yes, the definite form of night is necessary in Danish. In Danish the lack of the definite form would change the meaning, if you write it as 'a night' 'en nat' it is only one night, either one specific night or one night at some non-fixed point in time. Native Danish speaker by the way :)
Some Ninjas work at night. The ones that do work at night do it because the night shift pays double time, as there are also day time Ninjas, but they do more clerical duties. The double time also reflects the extra risk they face as they actually have to perform missions and stuff.
There is a bug report form for issues that aren't to do with the sentences specifically. Unfortunately it's a bit hidden away: https://support.duolingo.com/hc/en-us/requests/new. Make sure to add a screenshot to make the issue clear.
Of course issues to do with the grammar or corrections to the sentences themselves should still be done on the "Report a Problem" option in the lessons themselves.