Sometimes add partition will not add placement rules in PD

[lcwangchao]

CREATE TABLE tp (id INT) PARTITION BY RANGE (id) followers=1 ( 
        PARTITION p0 VALUES LESS THAN (100),
        PARTITION p1 VALUES LESS THAN (1000)
);

ADD PARTITION (PARTITION p2 VALUES LESS THAN (10000));

The new added partition will not have the placement rules synced to PD