When I look at the screenshot it doesn't look like a PowerCenter mapping to me.
Looks more like a process developed in Developer client.
Please redirect this thread to appropriate product as this doesn't seem to be PowerCenter related.
Can you please share the file size that you are looking to transfer. Please re trigger the task by configuring "SFTP_MaxWriteSize " Value as "8192" and share the outcome of the file transferred and session log for anlaysis.
With the SFTP_MAXWRITESIZE set to 8K
We attempted to send 116245 rows
the # of rows that were sent was 98,901
This was described in the original problem description
You can run the session with verbose mode ,which will help in understanding the place where the value is getting truncated .
Since you are using the pass through expression in between as shown over the document attached.
I will rerun with Verbose mode enabled and provide the logs.
Just to be safe, Please provide the steps to enable Verbose mode and the log files required.
To run a mapping in verbose mode you have to create MCT using the mapping and enable verbose mode in schedule step of the MCT. You will have to collect the session log after enabling verbose mode.
We see few errors capture in the session logs and one of the cause is if the FTP server needs the file to be transferred in multiples of bytes. For instance, 8 kb, 32 kb, and so on.
WRITER_1_*_1> CMN_1053 [2020-11-20 06:34:31.992] SFTP write failed. [-37]  
WRITER_1_*_1> CMN_1761 [2020-11-20 06:34:32.040] Timestamp Event: [Fri Nov 20 06:34:32 2020]
WRITER_1_*_1> WRT_8179 [2020-11-20 06:34:32.040] Error occurred while trying to close output file [EPICOR_OpenAR_Hist_Export_csv]
We see you have already tired setting the option "SFTP_MaxWriteSize" but after setting have the agent restarted happened also for 32KB chunk file did you notice any additional details captured in your tomcat logs why it fails to process that data from secure agent any resource constraint.
Also additionally here we would suggest you to use file processor connector to see if that helps to process the complete file.