Incorect Display Size for decimal column

To repeat.
OK_SIMPLE_STMT(Stmt, "SELECT cast('123456789.5678' as decimal(20,4))");

CHECK_STMT_RC(Stmt, SQLColAttribute(Stmt, 1, SQL_COLUMN_DISPLAY_SIZE, NULL, 0, NULL, &Size));
/* 20(precision) + sign + decimal point */
is_num(Size, 22);

For some reason connector erroneously returned constant value for decimal types.