dc offset in a miller integrator
There's no offset. It works exactly as it should work. Your integrator charges at the positive half cycle of your V1 and discharges back at the negative half cycle.  this is the ideal operation.

In practice the integrator drifts due the leakage currents or opamp offset voltage. It can get stucked to max or min output voltage limit or it can gradually forget the starting point and operate as you probably waited. (= zero average due the leak resistance over the capacitor)

In practical integrator circuits there exists a reset switch or an inserted resistance in parallel  with C1.

ADDENDUM: Imagine you have a full glass of water. Take a straw and start to soak. The surface of the water sinks. Spit every drop back. The surface returns exactly to its initial position if you do not add anything of your own and do not forget to spit something back. That's a hydromechanical equivalent process for the behaviour of your circuit.