This issue isn’t limited to my NAS, apparently. I restored the same 4GB directory directly to the root file system on the server running Duplicati and as expected, it worked fine a second time, no errors, all files are an exact match to the “live” files on the backup source. So far so good.
Then I copied those files to my NAS using Linux “cp -a” to see if something in the network transfer between the server and the NAS was creating problems. After the files copied over I compared them to the live files, and they were again a perfect match. I did this test twice, and both times all the files copied directly from the server to the NAS perfectly.
Finally, I created a share on a Windows machine, mounted it on the server running Duplicati, and ran a restore operation to that share. 15 files had errors, and most were even-megabyte final file sizes, all truncated from the correct file sizes in the source.
My conclusion is that from my server, Duplicati isn’t able to reliably write to CIFS/SMB shares (whether they are Windows or Linux shares). I switched all my shares to NFS, and everything works perfectly, so I’m going with that for now.