Welcome to the forum @egtrev
Backing up to an SMB destination seems to be a common source of troubles. Sometimes they’re very solid (at least one person moved to NFS for reliability), other times the suspicion is that an intermittent connection (e.g. laptop moving around) contributed. Your file was believed to be uploaded, but wasn’t.
Although zero is a special number, sometimes files are non-zero-short with a round size inhexadecimal. Although this has been observed, I’m not sure if a reproducible case has been available to be looked at (and even if that were the case, the conclusion might be that Duplicati did OK, and SMB dropped data).
Is Duplicati on Windows, Linux, or something else? There might be some client-side tweaks to help this.
Please verify the sha256 hash? helped Linux, and Windows has some options. What’s the destination? Unfortunately I’m not personally familiar with all the tricks to help SMB, but I can see that some exist…