The OpenFOAM data writer does not write the face values associated to the fixedGradient boundary condition. If ParaView is loaded with Openfoam libraries (i.e. ParaFoam), ParaView correctly evaluates the patch values according to the provided gradient.
However, using the internal reader (.foam) and without loading OpenFOAM libraries (e.g. ParaView in windows), the gradient provided in the boundary condition is not considered and the patch values will be evaluated with zeroGradient assumption and hence the displayed patch values are wrong. In the other words, it displays same values for face values (at the patch) and the adjacent cell center value.
I recently reported to Openfoam issue tracking (here) but since the reader provided by OpenFOAM works properly, no update has been made to the code (e.g. writing the patch values in the data file so ParaView can easily read it).
I have already fixed this issue for my own need by modifying OpenFOAM code and force it to write the patch values.
However, I am reporting this here to see if there is any possible solution to this issue from the ParaView’s side so that other Openfoam/ParaView users will not face this issue in the future. ParaView does not even issue any warning about this so it will be assumed that the patch values are correct (we have been looking at wrong values for a while before figuring this out).