When inserting values into a table with auto_increment column via bulk operation, the result of mysql_stmt_insert_id() differs:
In case buffer_type MYSQL_TYPE_NULL is used, or indicator variable was set to STMT_INDICATOR_IGNORE the values of first inserted row will be returned.

Output from attached test case:

 Insert id with buffer_type MYSQL_TYPE_LONG: 2

 Max value for t1.a=2

 Insert id with buffer_type MYSQL_TYPE_NULL: 1

 Max value for t1.a=2

 Insert id with buffer_type STMT_INDICATOR_NULL: 1

 Max value for t1.a=2