When preparing to send AppendEntries, followers that lag behind last_snapshot_idx will not have a snapshot sent instead of AE. However, this state will persist as not sending AE means there's also no way to track their current index.

Currently the way to deal with this is for the application to manually call raft_node_set_next_idx() after snapshot has been installed. I think ideally the library should deal with it, although this may be good enough if documented.