It seems to be a bug; however, please note it is not recommended using multi-threading to download or upload files to DriveHQ server; (It is discouraged to use download manager to download files hosted on DriveHQ FTP as it can be used as a denial of service attack.).
We will correct this problem in the next version FTP server. In the meantime, please use no more that 2 threads (connections) when using a sub-user to logon. Creating many threads / connections won't really speed up the transfer.
It is strongly recommended using DriveHQ FileManager to download / upload files. It is faster than regular FTP; and it has a lot more features than regular FTP: file/folder sharing, group collaboration and automatic folder synchronization features, etc.