I noticed that on labvirt1007 (a stretch host using the openstack-mitaka-jessie component), facter was flagged for downgrade (from 2.4.6-1 to 2.4.6-1~bpo8 from the mitaka component).
It should be removed from there; facter 2.4 for jessie is already present in jessie-wikimedia/backports and this potentially clashes with the facter 3 migration as well.