I was of the mind originally its aero, but the car looked fine in barcelona, unless there is more energy going into the tyres on that circuit compared to monaco to hide that weakness.
Pat Symonds is suggesting that the surface of the tyre is warm but not enough energy is going through to bring the core of the tyre up to temperature for it to be in the performance window. I think his opinion holds some merit given that williams suffered greatly with this issue in the past 3 years or so.
This may well be setup related or aero related. But i am more inclined to believe it is setup, seeing as though bottas didn't have such a bad run as lewis around monaco; though the car still need an extra lap to bring up the tyre temperatures.