Compiler V6.12 Bug?

Simulation of following code, compiled with V6.12 and optimzation level -O2

#include <stdio.h>


typedef struct {
  char data[23];
} device_t;


static device_t deviceList[100];


static device_t* rotate(device_t *device)
{
  int index;

  index = (device - &deviceList[0]) + 1;

  if (index < 0 || index > 30) {
    index = 0;
  }

  printf("index = %d\n", index);
  return &deviceList[index];
}


int main(void)
{
  unsigned int n = 0;

  while (1) {
    printf("n = %d: ", n);
    rotate((device_t*)(n));
    n += 1000;
  }
}

prints

n = 0: index = 0
n = 1000: index = 0
n = 2000: index = 0
n = 3000: index = 0
n = 4000: index = 0
n = 5000: index = 0
n = 6000: index = 163395741
n = 7000: index = 350133493
n = 8000: index = 536871245
n = 9000: index = 723608997
n = 10000: index = 910346749
n = 11000: index = 1097084501
n = 12000: index = 1283822253
n = 13000: index = 1470560005
n = 14000: index = 1657297757
n = 15000: index = 1844035509

into Debug (printf) Viewer, which is obviously incorrect.

Maybe a bug, or do I missunderstand something?