The master/tasks_unreachable metric is incremented for each task that was running on an agent that is marked unreachable. However, this metric is never decremented when/if the agent re-registers. Hence, if an agent is repeatedly marked unreachable, the metric will continually increase.
Basically, this metric is actually recording the number of times that this master has marked a task unreachable. This is inconsistent with some other metrics: for example, master/tasks_running reports the number of tasks that are presently unreachable.