fseek() question with SD card

Hello!
Is fseek() fully supported on SDcard with RL-FLASH 4.12?

fp = fopen ("test.txt","ab");
printf("pos:%d\n",ftell(fp));
fseek(fp,4,SEEK_SET);
printf("pos:%d\n",ftell(fp));
fprintf(fp,"ABCD");
fclose(fp);
output:
pos:12
pos:4
but it only appends ABCD to the end of the file.

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