This is by design. The transfer is resumed if aborted but it cannot resume a partly transferred file if you didn't use multi part transfers before. If the size of the remote file is suddenly different from the saved size in the metadata the file will be restarted from 0. These are some of the reasons why I recommended not to use multi part transfers. You need to understand a bit of the concept before you transfer every file in multiple parts. It is only recommended for large files that do not change on the remote server.
But maybe we can improve some specific situations. Could you please describe your scenario in detail?