Currently, if a JournalManager crashes and misses some segment of logs, and then comes back, it will be re-added as a valid part of the quorum on the next log roll. However, it will not have a complete history of log segments (i.e any individual JN may have gaps in its transaction history). This mirrors the behavior of the NameNode when there are multiple local directories specified.
However, it would be better if a background thread noticed these gaps and "filled them in" by grabbing the segments from other JournalNodes. This increases the resilience of the system when JournalNodes get reformatted or otherwise lose their local disk.