Because Duplicati only uploads changes to files (which might be initial creation), bits of files get spread around in many dblock files. I think the restore method doesn’t redundantly download a given dblock file every time it needs blocks from it, but distributes all the needed blocks whenever it downloads a dblock.
What this means is that the restore happens in pieces, as file blocks and timestamps get onto the drive. Just looking at the file sitting around might not show that it’s not fully there yet, though some of it is there.
MS SQL does not backup modified files explains how one can use –disable-filetime-check to help there, however I’m not sure of its implications, e.g. does it have to look through every file that is in source area? Possibly having a very limited private backup for troublesome files would be a workaround, if necessary.