Trouble with MinGW's mkstemp

The simple test program below almost always fails on my system.
"Almost always" means that it succeeds to create and open a temporary
file once every 6 invocations, on the average, and fails for all the
rest.  When it fails, the value of errno is mostly EINVAL, but on a
couple of occasions I saw ENOENT.

If I step with a debugger into __mingw_mkstemp, I see that
__mingw_crypto_tmpname in most cases generates a file name that is
invalid on Windows filesystems: it includes characters whose value is
below 32, and trying to create such files fails with EINVAL.  I'm
guessing that the few cases where I saw ENOENT were because the
generated file name included characters above decimal 31, but those
which are not allowed, like a slash or a backslash.  And since
__mingw_mkstemp only retries with a different file name if the error
is EEXIST, it bails out immediately after trying such file names.

This is with the latest MinGW runtime 5.2 on a Windows XP system, with
MinGW GCC 7.3.0 and Binutils 2.31.1.

Did I miss something obvious, or is this a real bug?

Here's the test program.  Invoke it with a single argument that is the
base-name part of the template to be passed to mkstemp, like "XXXXXX"
or "fooXXXXXX".

#include <stdio.h>
#include <stdlib.h>
#include <string.h>

int
main (int argc, char *argv[])
{
  if (argc > 1)
    {
      char tmpl[FILENAME_MAX];
      char *tev = getenv ("TMPDIR");
      if (!tev)
	tev = getenv ("TEMP");
      if (tev)
	{
	  int fd;
	  strcpy (tmpl, tev);
	  strcat (tmpl, "/");
	  strcat (tmpl, argv[1]);
	  fd = mkstemp (tmpl);
	  printf ("%s => fd: %d\n", tmpl, fd);
	}
      else
	fprintf (stderr, "No usable TMP variable!\n");
    }
  return 0;
}

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