Preconditions:

foo.h

#ifndef FOO_H
#define FOO_H

class Foo {
public:
    Foo();
    void func1();
    void func2();
    void func3();
};

#endif 

foo.cpp

#include "foo.h"

Foo::Foo() {}

void Foo::func3() {}
void Foo::func2() {}
void Foo::func1() {} 

Steps to reproduce:
1. In the header file context menu select 'Refactor -> Re-order Member Function Definitions According to Declaration Order' -> functions are re-ordered correctly in the cpp file
2. Restore to the preconditions
3. Delete the line 'void Foo::func3() {}' from the cpp file
4. Re-order the functions -> re-ordering works OK
5. Restore to the preconditions
6. Delete the line 'void Foo::func1() {}' from the cpp file
7. Re-order the functions

Result:
Functions are not re-ordered.