JDBC option "sessionInitStatement" does not executes sql statement when write data into database from dataframe

I need to write dataframe data into database, after database session is opened to the remote DB and before starting to write data, I need to execute a custom SQL statement with option "sessionInitStatement", all i know is that "sessionInitStatement" will always executes the configured sql statement before operation.

When write data with code below:

df.toDF(columns: _*).write.format("jdbc").options(
 Map(
 "url" -> db.url,
 "user" -> db.user,
 "dbtable" -> table,
 "password" -> db.password,
 JDBC_BATCH_INSERT_SIZE -> batchSize.toString,
 "sessionInitStatement" -> "set @@session.tidb_batch_insert=1;"
 )).mode(Append).save()

the sql statement "set @@session.tidb_batch_insert=1;" is never executed.

If i read data from database:

reader.format("jdbc").options(
  Map(
    "url" -> dbProperties.dbUrl,
    "dbtable" -> alias(queryStatement),
    "user" -> dbProperties.userName,
    "sessionInitStatement" -> "set @@session.tidb_batch_insert=1;",
    "password" -> dbProperties.password
  )).load

the sql statement "set @@session.tidb_batch_insert=1;" is executed correctly.

In my opinion, "sessionInitStatement" option should execute sql statement after database session is opened to the remote DB, no matter you read data or write data.