Rcpp::String keep UTF-8 encoding, but std::string does not

Question

How do I interact with code that builds a vector of strings with std::vector<std::string> and maintain UTF-8 encoding in the same way that Rcpp::String does by default?

I have a std::vector<std::string> of UTF-8 strings that I want to be able to return to R. Rcpp wrap()s as expected, returning a character vector, but it appears to drop the UTF-8 encoding (on Windows). I'm assuming this is caused by R's underlying string behavior, but if an Rcpp::CharacterVector is built using Rcpp::Strings, the behavior is correct.

Here's an example using a std::vector<std::string>...

#include <Rcpp.h>

// [[Rcpp::export]]
std::vector<std::string> cpp_foo() {
  std::string let1("ف");
  std::string let2("خ");

  std::vector<std::string> out;
  out.push_back(let1);
  out.push_back(let2);

  return out;
}

Which mangles the strings.

cpp_foo()
# [1] "Ù\u0081" "Ø®"

Here's an example of the desired behavior, using Rcpp::String...

#include <Rcpp.h>

// [[Rcpp::export]]
Rcpp::CharacterVector rcpp_foo() {
  Rcpp::String let1("ف");
  Rcpp::String let2("خ");

  Rcpp::CharacterVector out;
  out.push_back(let1);
  out.push_back(let2);

  return out;
}

... which preserves the strings.

rcpp_foo()
# [1] "ف" "خ"

sessionInfo()
# R version 3.6.1 (2019-07-05)
# Platform: x86_64-w64-mingw32/x64 (64-bit)
# Running under: Windows 10 x64 (build 18362)
# 
# Matrix products: default
# 
# locale:
# [1] LC_COLLATE=English_United States.1252  LC_CTYPE=English_United States.1252   
# [3] LC_MONETARY=English_United States.1252 LC_NUMERIC=C                          
# [5] LC_TIME=English_United States.1252    
# 
# attached base packages:
#   [1] stats     graphics  grDevices utils     datasets  methods   base     
# 
# loaded via a namespace (and not attached):
#   [1] compiler_3.6.1 tools_3.6.1    Rcpp_1.0.2     packrat_0.5.0