As mentioned by myself and others before, Pep seems to like this 1 defensive fullback and 1 offensive fullback against teams who sit back (he splits the players, 6 attack vs 4 defend, versus his normal 5-5 split):
Mendy and Walker, or Cancelo and Ake
The "problem" if you can call it that is that our best offensive fullback (Cancelo) and defensive fullback (Walker) do not play in the same pairing based on above...
Well they have, Pep has played Cancelo LB and Walker RB many times because he clearly sees them as our best fullbacks as many of us do, and he is forcing them on like this.
It's not ideal, but Cancelo is still decent at creating on the left (many stats show this and we can see too), but clearly on the right we see more of things like Cancelo making through ball passes to Sterling time and again, as well as crossing to Aguero's golden chance, things that are more natural on the right side for him as he's right footed.
Somebody mentioned just putting Walker as a LB to solve it, but Walker is our least proficient defender on the ball, playing him on his weaker foot seems like a bad idea, and probably why Pep does what he does, this means our best combos are usually (due to an unreliably fit Mendy):
1) Cancelo LB, Walker RB
2) Ake LB, Cancelo RB
Which means Cancelo is a must to have maximium effectiveness, and I could see going forward, Pep almost having to play him every game because teams aren't pressing as effectively like normal seasons.
Playing Ake and Walker together is probably TOO defensive, but maybe preferred by Pep against top teams as he may want to defend with 5 (2 CBs + 2 FBs tuckin in + 1 DM) versus 4 (Shifted back 3 + 1 DM)