It depends on how SMB is configured and the underlying filesystem. SMB will translate as best it can Linux permissions to something that Windows understands. It’s not a perfect translation due to inherent differences between Windows and Linux. Some concepts just don’t exist in Linux (such as a file’s “archive” bit). On modern systems these Windows-only properties are stored as extended attributes on the Linux filesystem.
Here’s an example from one of my Linux systems that has an SMB share. I placed a file on it and tweaked its file attributes:
$ getfattr -d Test.txt
# file: Test.txt
user.DOSATTRIB=0sAAAEAAQAAABRAAAAIAAAAOavCfVIkNcB5q8J9UiQ1wE=
Unfortunately my Synology doesn’t have the getfattr command so I can’t check there, but I’m almost certain it is making use of extended attributes.
In any case, Duplicati doesn’t see any of that … the SMB layer just dutifully translates everything as best it can and to Duplicati it looks like it’s seeing a regular DOS/Windows based system. So Duplicati will be storing Windows-style metadata.