If they have not been both told to get forward at all times then they are ignoring instructions from the management for the whole season as that is exactly what they have been doing all season.
It was deliberate to take in a more attacking left back to allow Iniesta or whoever is on the left to play further inside and closer to Messi.
The downside of that is that the defence looks so open and teams like Madrid and Milan are almost just blindly hitting the ball out wide when they counter and getting opportunities from it.
Why not just allow one full back to go up at one time? That way there can be three at the back at all times.. The most dangerous flank could have it's fullback stay in a defensive position. Eg Alves could defend Ronaldo's flank and Alba can join the attack or vice versa depending on the threat.
Although we play such a high line coupled with Pique's adventurous nature that Ronaldo and Di Maria can always get in behind the defense.