Campagnolo Athena 11s front-derailer alignment
I have the same model front derailleur and have experienced the same issue.

It is unlikely that the bike is bending enough under pedalling force to cause this (unless the frame is broken or cracked). More likely it is a problem with the front derailleur, and the extra chain tension from pedalling hard exacerbates it.

Now, obviously if there is damage such as a bent front derailleur cage then this could affect the shifting. But if the problem has just started happening from normal use, without the bike being involved in any recent crashes or mishaps, then you probably just need to service your derailleur.

The manual has a section on maintenance that if followed should resolve the problem. Here are the main points along with some of my own observations:


Check the derailleur is clean and moves freely. When I suffered this problem there was grit in the moving parts, preventing the cage from moving all the way to the lower lower stop when downshifting. One way to verify the smooth action of the mechanism is set the derailleur to the small chainring and carefully pull at the cage with your fingers. It should move smoothly, with the spring pulling back quite firmly against you towards the seatpost. I find spray lubricant (e.g WD-40) to be very effective at displacing grit and freeing the mechanism, followed by a drop of oil at each pivot and some grease on the spring to keep it lubricated.
Check the shifter cable tension. Excessive tension can prevent the derailleur from sitting tight against the lower stop.
Ensure the derailleur is correctly aligned. You mention that there is an “increase in gap” at the rear.. I can't gauge how significant this is without seeing it, but the manual does state that the outside of the cage must be parallel to the chainrings. If it is too far out of alignment it can definitely affect the shifting. Also check the vertical alignment: the gap between the large chainring teeth and the cage should be 1.5mm to 3.0mm.
Check the adjustment of the lower limit screw. With the chain on the small chainring and largest sprocket, there should be a 0.5mm gap between the chain and inner face of the FD cage. I find it is best to do this with a bit of tension in the chain, as sag in the chain affects the size of the gap there.