Hi,
is this potentially not a visualization issue where the the value is referenced to some normal direction? Which structural solver is this? What could be happening in the structural solver is the value is given in some direction relative to the face normal. If this is the case, when the normal is facing outwards from the structure, and the force is going inwards, it could be taken as negative and that could be what is being plotted. Therefore the force is negative to the face normal, but still in the correct direction.
To check if the actual forces being sent from preCICE are correct, you could plot the values in the x,y, z direction and check that the direction of the forces during the simulation runtime are actually correct? If the structure is moving correctly (from initial looks), that is a good sign.
Regards,
Kyle