Windows Wow64 scheduler randomly alters FS segment register

/*      
When the WoW64 scheduler does a context switch and then returns control to this
test program, modifies the fs segment register. The frequency of this
modification depends on the CPU load. 
I usually start 4 cmd prompts running on each any program that uses CPU. I start
4 of them to use the 4 cores on an i3 CPU.

Win7_64 and Win10_64 have this problem.
WinXP_32, Win7_32 and Win10_32 work ok.

On this test program I copy ds to fs, and then read back the fs.
If it is not the same as ds, I print an error.
Every 10^7 loops I print the loop count.

cl test_fs_seg.cpp
        
*/
#include <stdio.h>
#include <windows.h>

/* ---------------------------------------------------------- */
void main()
{
        unsigned dds, ffs, i;

        // If I add this line, the bug frequency reduces a lot
        //SetThreadPriority( GetCurrentThread(), THREAD_PRIORITY_TIME_CRITICAL );   

        _asm {
                xor    eax, eax
                mov    ax, ds
                mov    dds, eax
        };

        for ( i = 0; i < 2000000000; ++i ) {
        
                _asm {
                        push   fs
                        xor    eax, eax
                        mov    ax, ds
                        mov    fs,ax
                        mov    ax, fs
                        mov    ffs, eax
                        pop    fs
                };
                if ( ffs != dds ) {
                        printf( "fs:%x  ds:%x\n", ffs, dds );
                }
                if ( (i % 10000000) == 0 ) {
                        printf( "%d\n", i );
                }
        }
}
/* ---------------------------------------------------------- */