Premise: I am not sure whether I am missing something theoretically.
Given a circuit creating a GHZ state over 3 qubits, say q1, q2 and q3. If I do not consider q3 and perform a state tomography over q1 and q2, then intuitively, I should get a Bell state as output, i.e. $|\Phi^+\rangle\langle\Phi^+|$.
Nevertheless the output is the following matrix:
[[ 0.5, -0. , 0. , -0. ], [-0. , 0. , 0. , -0. ], [ 0. , 0. , 0. , 0. ], [-0. , -0. , 0. , 0.5]]
I am not observing q3. I also tried to reset it with the command
This is how may look one of the 9 circuits prepared for the tomography:
EDIT: here there is a partial answer, however, I'd still like to understand what is the meaning of my output matrix.