Djimon Hounsou’s character in “The Longest Day in Chang’an” should’ve been dubbed by an African-Chinese (they exist!) person, not only for representation’s sake but also because Black people have distinct voices that other races don’t have. That’s my two cents.
I mean...the point of dubbing in drama is actually usually to homogenise accents and voices, not to create racial/locational distinctions. So...
ETA: Okay just so I don't come off as being flippant about this. I honestly think that you have to manage your expectation regarding things like this in cdramas. Aside from their entertainment value, dramas are a tool of propaganda in a country like China, which is the reason why censorship is a thing and there is such a controversy around Zhang Linghe's pretty boy general image recently that even the CCP got involved. Why do you think CCP cares what fictional generals look like in a drama? Because it reflect the trends and definitions of masculinity in real life, and China does not want to create or convey an image of its men being in any way feminine, which is why CCP has to condemn pretty actors wearing make up playing fictional army generals in an idol drama.
Being aware of that, you can't expect this kind of racial/diversity representation, or even, for example, for a Hui actress to play a historical character like Consort Rong, because it goes against the messages that the government would like to promote. Yes, a drama like Longest Day in Chang'an is going to show the diversity of races that did historically exist in this period of the Tang dynasty, but that showcase of mixing races is more to serve the purpose of conveying how prosperous the Tang dynasty was (look how rich and powerful we were, even all these foreigners wanted to come and trade and live here), not to give opportunities and representation to actual racially-diverse actors, or to celebrate diversity in and of itself. Ethnic and racial diversity can exist in dramas because ethnically and racially diverse people have always existed, but they are more often than not played by Han actors (at least in terms of ethnically diverse characters) and dubbed by actors speaking standard accents, because the message is other ethnic groups are just lesser types of Han. Han supremacy and the assimilation/conforming of non-Han groups into Han identity is actually the point. This is why ethnically diverse actors like Dilireba rarely actually play non-Han characters. This is why they're not going to go accurate on the representation even in a drama where there is diversity like Chang'an, because the representation is not the thing that actually matters.
You can't expect more from cdramas than what they are allowed to be.