5-0-4 false positive?

[dg1980]

Dear MISRA team,

the following comparison of two values of the same type is marked as violation of 5-0-4:

Code:

typedef unsigned char ui8;

extern ui8 a;
extern ui8 b;

void test(void)
{
  if (a != b)// false positive?
  {
    a = b;
  }
}

Is this a false positive?
I fail to see implementation-defined behaviour.
BTW, the rule scope/text fails to reference the implementation-defined behaviour.
Thanks.