PIB2BSFTP Receiver SFTP - Append not writing the first line correctly?

[LouieReyes]

We recently updated to SP04 PL46 latest at that time of PIB2BSFTP. I tried using the APPEND mode but whenever it does try to append the first line is always resulting to "NULL" values. We are on SAP PI 7.4 SP13

Example:

1st line to be appended: 123 456 789

2nd line to be appended: 001 002 003

3rd line to be appended: 311 411 511

Result:

NULL NULL NULL NULL NULL NULL001 002 003

311 411 511

Any idea why this is happening?