IBM circuit function level 2 tries to extrapolate circuits with no 2Q gates

[nonhermitian]

Steps to reproduce the problem

Consider the circuit operator pair:

qc = QuantumCircuit(6)
op = SparsePauliOp('Z'*6)

both level 1 and level 3 return a value near one, e.g. 1.00725446 and 0.98866819, respectively. However, level 2 is very off, returning a value of 1.17241824, which is very off.

What is the current behavior?

See above. My guess is the routine is trying to extrapolate when there are no 2Q gates

What is the expected behavior?

Level 2 should not perform worse than execution with no additional mitigation