Hi, I have a similar problem.
As example, the structure is a basic dipole antenna, two cylinders linked by a short sheet where there is the lumped port. (same as the tutorials from Ansoft)
If I set an impedance boundary at the end of cylinders, on the circle surface in contact with the lumped port, zero current is flowing (all results are zero), but if the impedance boundary is on the all cylinder except the previous circle face, the current is flowing, results calculated and expected match quite good.
So, what's wrong with impedance boundaries in contact with lumped port ?