Such praise is of course racially-charged. Moreover, it carries implications that the individual being praised, or 'praised', is an exceptional specimen that proves the general inferiority of their racial-national type to the racial-national type of the praise-giver. In stories where themes occasionally toe the line of political correctness, the phrase is sometimes used with as much dripping sarcasm as humanly possible when someone does something egregiously idiotic. This article produced in the USA, home of much of the Western world's racialist discourse analyzes in-depth all the implications of such statements.
In a contrast, Values Dissonance may come into play that may not be fully understood by either the audience or the characters within the story. History has seen many debates, and conflicts, over the nature of race and ethnicity. In the past two centuries, in particular, Scientific Racialist Theory bloomed and then withered alongside Phrenology and Eugenics and Social Darwinism. The (first half of the) twentieth century was a time of intense ethnic conflict due to the racial-isation of world culture, which had also become fixated upon concepts of 'racial degeneration' and 'racial purity'.
In the post-war period, the credibility of all those movements was dealt a death blow by its association with Nazi Germany and what it did. The decades since have seen European people come to accept the idea that they are not God's Gift To Humanity, and to treat their fellow man with a measure of respect. Thus, although we did say that You Are a Credit to Your Race-type praise was indicative of racialist beliefs, the fact that they praised a character of a different ethnicity shows that they have begun to accept them (as an individual or as a 'race') as being worthy of a measure of praise. If only a teensy bit. So while it could just be a "you've impressed me/received my blessing" kind of statement, it could also be a "you've surprised me/caused me to revise upward my ideas of what your 'race' is capable of." This often shows up in works that are Fair for Its Day.
Even in modern societies where political correctness is the order of the day, this trope hasn't gone away. In addition to the classic "Ignorant Idiot Unaware of the Unfortunate Implications" or "Actual Racist Spouting a Transparently Back-Handed Complement", we now have the "Be an Ally to [Insert Minority of Choice Here]" movement. As well-intentioned as it is to teach people to be mindful of the hardships suffered by minorities (and that those in the majority may have unintentionally benefited unfairly from such injustices), it's all too easy for there to be the unfortunate implication of "Be One of the Good Ones" (just directed at [Insert Majority of Choice] rather than the other way around), and a lot of times the topic at hand is lost in the debate over whether or not the sentiment was on purpose.