This seems to be fairly repeatable, at least in our current setup.
If a node is rendering a frame and either loses its network connection or is shut down (ie, power outage, Pulze task crashes, system update…) then that frame is permanently lost from the job queue. Resetting the job does not recover it, and it does not re-appear in the queue after the node has been reconnected. In some cases, all frames that the node had worked on are flushed from the queue.
In the moments after the node goes offline its tasks still appear correctly in the job view. The “Nodes” view then cascades from “ONLINE” through several status messages before settling on “OFFLINE”. At this point, some or all of its tasks will disappear from the job view.