mysql binlog file size stop growing

Question

I made a master(A)-slave(B)-slave(C) mode MySQL instances for backup. But today when I see whether the mode works fine, I find A and B are OK. That is B can get binlog from A, but B does not write updates to its binlog. So C cannot sync with B.

After some investigation, I see that B's new binlog file is 120 bytes and there are not any errors in MySQL's log. The sql_log_bin is ON. So I don't know what can be the reason that causes the problem. What I can supply as an addition is that the new binlog's create time is when I made a mysqldump on B. So I guess there may be some relationship between the mysqldump event and the binlog write event? Hopefully someone can help me out, thanks a lot.

B's master status:

mysql> show master status;
+-----------------+----------+--------------+------------------+-------------------+
| File            | Position | Binlog_Do_DB | Binlog_Ignore_DB | Executed_Gtid_Set |
+-----------------+----------+--------------+------------------+-------------------+
| mysqlbin.000016 |      120 |              |                  |                   |
+-----------------+----------+--------------+------------------+-------------------+

B's binlog file size status:

ll /data/mysql/
-rw-rw----. 1 mysql mysql        166 7月   7 02:00 mysqlbin.000015
-rw-rw----. 1 mysql mysql        120 7月   7 02:00 mysqlbin.000016
-rw-rw----. 1 mysql mysql        288 7月   7 02:00 mysqlbin.index

B's mysqldump SQL:

mysqldump --master-data=2 --all-databases --flush-logs --user=xxx  --socket=/tmp/mysqld.sock -A > /tmp/backup.sql