We allow the creation of logical sessions on arbiters. These sessions are distinct from sessions on the rest of the replica set or sharded cluster; they are strictly local.
However, the logical session reaper does not work on arbiters. Since
SERVER-40441, it is disabled explicitly. Before that, it would fail because config.system.sessions does not exist on arbiters (and even if you created it, it wouldn't be sharded on a sharded cluster). This results in eventually running out of space in the session cache, if any are being created.