When downloading a group of files, sometimes the resulting TAR file can contain empty files with a .MISSING extension. What this means is that the Flywheel server was unable to successfully retrieve the file at the time of the download attempt. The server generates these evidence files, rather than fail the download outright, because often times, users would rather immediately use the rest of the files which are available.
If you encounter one of these files, these are the things you can do.
Attempt the same download again. This may not be practical when downloading large multi-GB datasets.
Attempt to download the individual .MISSING file(s)
Report the issue to Flywheel Support <support@flywheel.io> if multiple download attempts fail. Make sure to include the full path (all enclosing folders) to the .MISSING file, as that will enable a quicker investigation and resolution.