https://bugs.winehq.org/show_bug.cgi?id=42412
--- Comment #41 from Bruno Jesus 00cpxxx@gmail.com --- I'm not saying you should dump WinUAE, it is clearly the most talked about Amiga emulator. I'm saying that you could try alternatives to check if the problem is also present in other emulators.
The magic of TCP is that it takes care of a lot of common problems in data transmission like lost packages, out of order and congestion control. So it is very common in Wireshark to see black lines of out of order or resent packages.
A different approach would be to write a server application that sends a well-known set of data (eg 2048 bytes of each alphabet character) and checking if a client inside Amiga receives the data correctly. If the data is received correctly the problem is the FTP application.
Another approach would be to run Wireshark inside Amiga.