I have found this bug recently that currently, the ensemble just doesn’t fulfil the intended purpose of purchasing other class elite sets once Rival was earned on one character.
The problem is that one needs Rival earned in the respective DF seasons, for which one unlocks the elite PVP set already, making the ensemble useless.
More in concrete, if I earn 1800 CR in any rated bracket on my rogue, I receive the elite set for rogue already, and makes the ensemble pointless, as I already earned it.
I believe the whole point is with the warband feature and account-wide achievements is to allow people who earned Rival on any of their characters in their Bnet account,to be eligible to purchase the elite sets for the other classes as well.
I hope the development team can look into this issue and provide a fix, or removal of the ensembles, as in their current state they are really not doing anything useful.