This issue is reproducible through a Jepsen test of materialized views that crashes and decommissions nodes throughout the test.
In a 5 node cluster, if a node crashes at a certain point (unknown) during the decommission of a different node, it may start with a null entry for the decommissioned node like so:
DN 10.0.0.5 ? 256 ? null rack1
This entry does not get updated/cleared by gossip. This entry is removed upon a restart of the affected node.
This issue is further detailed in ticket 10068.