"Have a good evening Rehema"
Translation:Jioni njema Rehema
I suppose it could be both. But the translation, it feels like it is missing something. Jioni means evening, njema means good/fine/well. The word 'Have' is not described. So both farewell and greeting in this case.
Is this because jioni and asubuhi are a different case than usiku and mchana? Also, why is "mzuri" not accepted?
Not cases, but noun classes. Swahili doesn't mark nouns for case. Instead it marks the subject and object of the sentence on the verb.
But, yes the point is still valid. They belong to different groupings of nouns, and therefore demand different concord markings on the adjectives.