While writing the data to a pre-existing table in the unity catalog using JDBC. it just writes the Delta of the data. 

Driver used: 

Lets say I have the table has rows:

+-+-+
|a|b| 
+-+-+
|1|2|
|3|4|

 and I am appending the row using .union

+-+-+
|a|b|
+-+-+
|1|2|
+-+-+

I am using a `.union` here after reading the table with the row to append which results to this in my logs

+-+-+
|a|b| 
+-+-+
|1|2|
|3|4|
+-+-+

but after writing to table with mode ".overwrite" using jdbc

write_options = {"url": catalog_jdbc_url,
                         "driver": "com.databricks.client.jdbc.Driver"}
 (df.write.format("jdbc")
 .options(**write_options)
 .option("truncate", "true")
 .option("dbtable", table_name)
 .save(mode="overwrite"))

the table just shows in the unity catalog:

+-+-+
|a|b|
+-+-+
|1|2|
+-+-+

 Is there any reason for this or config issue?