If AF staff buy confirmed AF J tickets with an employee discount (I am pretty sure it exists and it is available to all employees) then these seats should really be considered as "confirmed". Paid upgrades should not result into an overbooked J cabin and get confirmed AF staff downgraded to W or Y.
If the union's idea is to keep available J seats to seat AF staff on leisure standby eco ticket, well, I would not be surprised but that is unacceptable.

But I think for medium haul, they found another technic: they make sure cash upgrades are prohibitively expensive, to keep available J seats for their Y stand by friends