That’s completely normal, and not a problem.
The FTP connection is shut down after 20 seconds of inactivity. But to state very clearly: this only happens on inactivity, it does not impact active transfers.
FTP works by setting up one connection to do file listings and other server commands (called the control connections) and other connections to handle actual file transfers (called data connections). When transferring many files (or big files), it’s possible for the control connection to be shut down while the data connections are still transferring.
Again, this is completely normal behavior and doesn’t affect your ability to upload files, because the transfers will keep running.
However, there are other reasons for your compressed files to fail to upload. This article has a few different reasons why it might happen.