Typically the Expected there to be a temporary fileset for synthetic filelist errors are related to the backup job having been interrupted. What destination are you using?
What are you seeing that makes you feel it has restarted?
Chances are the interrupted DID cause your backup to re-start, but the first thing it does is scan through everything to see what was already completed so it can pick up where it last left off.
Duplicati is pretty robust, you shouldn’t have to do anything and should figure out what to do on it’s own. However, if you want to try something you could add the --disable-synthetic-filelist parameter to your backup job and see what happens.
–disable-synthetic-filelist
If Duplicati detects that the previous backup did not complete, it will generate a filelist that is a merge of the last completed backup and the contents that were uploaded in the incomplete backup session.