For example, passing a string consisting of characters 80 and 80 with a second argument of 2 will can [sic] result in "\x80" (correct) and "\x80\x80" (incorrect).

The way you've worded this makes it sound like the output is not deterministic, which is certainly not the case. Also, "a string consisting of characters 80 and 80" is not specific enough for a test case. But please feel free to provide some actual test code that demonstrates the bug you are trying to explain, or better yet, show how you would've coded it to (at least in your view) "correctly" handle the different strings "\x80\x80" and "\N{U+80}\N{U+80}".