In fact, I think it's because I didn't realize gather would remove the source files using "rm". When using rsync in other situations, if I want to delete the source files, I would use "--remove-source-files" flag. It would only remove these files transferred. So when I check the rsync flags, I thought it was OK.
Unbalanced always works well, so I never read the log before. When I found some files were deleted, for the first time, I read the log and found that Unbalanced will remove the old files using "rm". Maybe it's a good idea to use "--remove-source-files" instead of another "rm"? I dont know if this would work for unraid file system.