Pig 0.15.0 cannot STORE the same alias onto HDFS and Mysql both?

I am using Pig 0.15.0 and have found that maybe it does not support STOREing an alias onto HDFS and Mysql both. the question is simplified as follows:

first, I have a data file on hdfs://tmp/file, which contains:

1046074327,40986
1473299786,1

then, I created a Mysql table db_test, whose schema is:

CREATE TABLE `db_test` (
`id` bigint(20) NOT NULL,
`cnt` bigint(20) NOT NULL
) ENGINE=InnoDB DEFAULT CHARSET=utf8;

then I have written a Pig script which runs in mapreduce mode on Hadoop 2.7.1, and the script contains:

REGISTER '/path/to/mysql-connector-java-5.1.38-bin.jar';

%declare DBHOST '127.0.0.1'
%declare DBPORT '3306'
%declare DATABASE 'test'
%declare USERNAME 'root'
%declare PASSWORD 'toor'

a = load '/tmp/file' USING PigStorage(',') AS (id:long, cnt:long);

STORE a INTO '/tmp/db_test2' USING PigStorage(',');

STORE a INTO 'db_test' USING org.apache.pig.piggybank.storage.DBStorage('com.mysql.jdbc.Driver',
'jdbc:mysql://$DBHOST:$DBPORT/$DATABASE?useUnicode=true&characterEncoding=utf-8',
'$USERNAME', '$PASSWORD',
'REPLACE INTO db_test (id, cnt) VALUES (?,?)');

however, the second STORE will never work, without any error reported. However, if I comment the first STORE line, the second STORE will work! What a magic!

I have tried to use Pig 0.16.0 in local mode on my own host and it can even not instantiate mysql:

Caused by: java.lang.RuntimeException: could not instantiate 'org.apache.pig.piggybank.storage.DBStorage' with arguments '[com.mysql.jdbc.Driver, jdbc:mysql://127.0.0.1:3306/test?useUnicode=true&characterEncoding=utf-8, root, toor, REPLACE INTO db_test (app_id, cnt) VALUES (?,?)]'
at org.apache.pig.impl.PigContext.instantiateFuncFromSpec(PigContext.java:770)
at org.apache.pig.parser.LogicalPlanBuilder.buildStoreOp(LogicalPlanBuilder.java:988)
... 17 more
Caused by: java.lang.reflect.InvocationTargetException
at sun.reflect.NativeConstructorAccessorImpl.newInstance0(Native Method)
at sun.reflect.NativeConstructorAccessorImpl.newInstance(NativeConstructorAccessorImpl.java:62)
at sun.reflect.DelegatingConstructorAccessorImpl.newInstance(DelegatingConstructorAccessorImpl.java:45)
at java.lang.reflect.Constructor.newInstance(Constructor.java:422)
at org.apache.pig.impl.PigContext.instantiateFuncFromSpec(PigContext.java:738)
... 18 more
Caused by: java.lang.RuntimeException: Can't load DB Driver
at org.apache.pig.piggybank.storage.DBStorage.<init>(DBStorage.java:82)
at org.apache.pig.piggybank.storage.DBStorage.<init>(DBStorage.java:71)
... 23 more
Caused by: java.lang.ClassNotFoundException: com.mysql.jdbc.Driver
at java.net.URLClassLoader.findClass(URLClassLoader.java:381)
at java.lang.ClassLoader.loadClass(ClassLoader.java:424)
at sun.misc.Launcher$AppClassLoader.loadClass(Launcher.java:331)
at java.lang.ClassLoader.loadClass(ClassLoader.java:357)
at java.lang.Class.forName0(Native Method)
at java.lang.Class.forName(Class.java:264)
at org.apache.pig.piggybank.storage.DBStorage.<init>(DBStorage.java:79)
... 24 more

The 'instantiate' problem may be due to my environment settings, and I will keep trying.

And can somebody help me with the 'two STORE' problem? Could it possibly be a bug?