Answer:
For the first point, the modulus of elasticity of the material is assumed in the material model, so that there is, so to speak, an initial state for the solver to ensure the numerical stability.
If no material is selected, the first point is not calculated as expected when creating the material model using the Diagram definition type, and cannot be adjusted reasonably.
To avoid this, you have to select the material in advance. Then, you can create the material model as usual, and adjust the points. The first point can now be adjusted with regard to the stress.
