Dear Robin,
I think I might find the issue.
The current way the code is organized, it reads the incoming date from the serial buffer, and writes the captured data to another serial buffer.
When the first data (between <>) is captured by arduino, it immediately print to teraterm, while the second data continues to fill the serial buffer.
I suspect if the first print takes too much time before finishing, and if the second data is larger than the serial buffer size, overflows of data will happen before they are read.
is this a possible explanation?
Now I just read once per data and write to my serial, and I achieved 1kByte of data correctly.