That is correct. The solution to this is given in section 5.4.1 (election restriction), section 5.4.2 (Committing entries from previous terms) and section 8 (Client interaction) of the Raft paper.
Roughly, a newly elected leader will have all committed entries (guaranteed by the "election restriction", 5.4.2) but it does not know precisely which are committed. The new leader will commit a no-op log entry (section 8) and after it has received replies from a majority of the cluster it will know which entries have already been committed.
