Hi @djmeee
Thanks for the detailed report.
I suspect that this is due to a 2D-3D mapping problem.
In your CalculiX coupling mesh, the y coordinate seems to be the span-wise axis (coordinates -0.0003 and 0.0003).
Currently, you ignore the z coordinate for your write mapping (option z-dead). You might need to change this to y-dead. And maybe you also need this for the read mapping.
Could you try to visualize both coupling meshes in paraview (fluid and solid, only the coupling meshes, so the meshes that preCICE exports) and upload a screenshot?
Is your span-wise width the same in OpenFOAM? Is your span-wise axis the same in OpenFOAM?
Also compare this discussion: 2D-3D Coupling OpenFOAM-DUNE
Benjamin