Resolved: Client is downloading all files when syncing unsynced folder once agian

Some of the libraries on my laptop have been uncynced automatically due to missing local folder (the disk was unavailable). After rebooting, I re-enabled syncing according to instructions in the docs ( Syncing With an Existing Folder).
To my surprise, it looks like Seafile client tries to download everything once again, even though all the files are present locally at the same locations. I was expecting it to do some control sum checking both on server and on client, but it just downloads everything.
Is this expected behavior?
If yes, is there a way to re-sync existing library without re-downloading existing files? The libraries in question are 1,5 TB in total and full download will take ages over a slow connection I have right now.

Seafile version is 9.0.13 pro
Client is 8.0.6 on Windows 10

Well, it’s kind of emarassing, but actually I was re-syncing wrong local path and that was why the client was downloading everything all over again.
It was a combination of my lack of attention, confusing GUI and very slow internet connection
When I was enabling syncing, I was switching between “sync with an existing folder” and “create a new sync folder” because the UI of this dialog always confuses me. And most likely I ended up selecting the correct path, but wrong option. This had happened to me before, so when the client changed status from “reading the list of files” to “downloading”, I double checked that there is no duplicated directory (e.g. c:\path\my-lib\my-lib). There wasn’t. But it has appeared much later (I suspect it took very long time to appear due to very slow internet connection).
Anyway, when I noticed all this, stopped the sync, deleted duplicate path, started the sync once again double and triple checking - everything worked as expected.

Leaving the original question and detailed explanation in hope that it might help some absentminded person like me, to find out what’s going on.