Thank you very much for your answer and reference paper!
From the tutorial I saw, I understood that the MIH material model did not accept negative slopes in its definition. That's the reason why I have input data only up to the necking point. Now I was reading through the Mechanical APDL manual, and I saw that this is really the case, except if you define the Material via APDL. The manual says that issuing the command TBEO ,NEGSLOPE,1 should do the trick, but it continues by creating the input table via APDL.
In my case I have a large set of data points, and I think this would be a really tedious task and also more prone to errors. Do you perhaps know if there is a way of defining after necking behavior (negative slopes) in the Engineering Data Source GUI?
As for using the 1/4 symmetry in the model, it resulted in the exact same behavior as the full model (which I think is a good think to be honest hehe). So the multiple necking regions are still there:
I would be really happy to hear further ideas on that also.