I use FTP a lot on a LAN and can test this later. Meanwhile here are some questions and suggestions.
1) Clarification: Are you moving files within a MiXplorer FTP server being accessed by another device (client) or are you moving files on a different FTP server being accessed by MXplorer as client?
2) Using a network meter while moving 2gb file within the same ftp server I can see clearly that the data is being transfered to my phone before it hits its destination.
3)And I can confirm that the the problem appears after the version 6.17.5.