The sequence that leads to Williams' goal starts with a common defensive issue under Koeman: Frenkie rushes forward to press, but teammates don't join him in the press. The opposition attacker (Muniain) then exploits the gap left behind Frenkie to comfortably receive the ball. https://twitter.com/AlexandraJonson/status/1350930209323823104

Another good example of this issue is Fede Valverde's goal in the latest Clásico.

I don't know what goes on backstage at Barça, but my guess is that unclear defensive instructions force players like Frenkie to rely more heavily on their own defensive intuition, which leads to a poorly coordinated and synchronized pressing that concedes lots of spaces.

It makes sense to rely more heavily on player intuition and instinct when building your attacking structure, but that doesn't work so well in defense. If players are not all rowing in the same direction, gaps will appear very easily in your defensive block.