GetEnvironmentVariableW and GetEnvironmentStringsW returns different values - c++

Question

I'm developing an UTF-8 API which should transparently work independent of the OS.
For that I'm replacing getenv by GetEnvironmentVariableW and the char** env passed to main by GetEnvironmentStringsW.
To make sure everything is correct I iterate over the env array and check each value against GetEnvironmentVariableW to make sure they match.
This works everywhere but with MinGW/MSYS where it fails for the env variable temp where getenv = C:\msys64\tmp and GetEnvironmentStringsW returns C:\Users\appveyor\AppData\Local\Temp\1
I verified the value using c:\msys64\usr\bin\env MSYSTEM=MINGW32 c:\msys64\usr\bin\bash -l -c "echo $temp" (similar how the build/test is run) and there it returns C:\Users\appveyor\AppData\Local\Temp\1.
So it seems GetEnvironmentVariableW returns a different/wrong value.
How is this possible? Can I avoid this?
Code of the test is:
for(char** e = env; *e != 0; e++)
{
const char* key_begin = *e;
const char* key_end = strchr(key_begin, '=');
std::string key = std::string(key_begin, key_end);
std::string value = key_end + 1;
const char* env_value = boost::nowide::getenv(key.c_str());
TEST_EQ(env_value, value);
}
where the getenv function is (reduced without error checking etc):
char* getenv(const char* key)
{
static const size_t buf_size = 64;
wchar_t buf[buf_size];
GetEnvironmentVariableW(widen(key), buf, buf_size);
return narrow(buf);
}
Edit: I was able to reproduce this with the following minimal code:
#include <iostream>
#include <windows.h>
#include <string>
int main()
{
const size_t buf_size = 4000;
char buf[buf_size];
GetEnvironmentVariable("temp", buf, buf_size);
std::cout << buf << std::endl;
return 0;
}
When compiled (e.g. in MSVC) and run with temp=Foobar ./a.out in e.g. Git bash on Windows it prints C:\Users\alex\AppData\Local\Temp not Foobar. In GetEnvironmentStrings the value is correct