Zinedine Zidane has a serious problem in the middle of defence. It doesn't matter which pairing he uses, there's a trend for them to drop back with huge gaps with the midfield. The midfield press with the forwards, but the centre-backs don't follow. That leads to a huge black hole between the two lines which rivals exploit.
It's a tactic which betrays Zidane, who has not been able to fix it. He does not know if it's due to a lack of work in training or if the defenders can't be bothered. He doesn't know what's going through the head of the players who drop back. It's strange that they've not been able to fix the unbalance. Only Casemiro has tried to address the instability.
The centre-backs also provoke a domino effect that moves them away from the midfielders and forwards and damages the connection between the lines due to the distance there is. Many times, the quality of passing disguises the errors, but if the passes fail, the opposition smell blood against a poorly positioned team.
The question is to find out why the defence is dropping so deep: if it's because of orders from Zidane or if it's because they're not following orders. Is it because of insecurity? Is it because they are not reading the game well? Are they scared of being caught out by opposition attackers?