Here is my take using the Orekit Python wrapper, a numerical propagator and the Eigen6s gravity field model limited to 8*8 spherical harmonics here. Using more harmonics has no visible effect on the plots anyways.
I chose to include a second definition of altitude (fourth plot), the altitude above a sphere whose radius is equal to Earth's equatorial radius. I find this quantity better to analyze the impact of orbit perturbations, because this pseudo-altitude should stay constant when there are no perturbations.
In the 90° inclination case, the peak-to-peak variation of the spherical altitude is around 8 kilometers. One can also see that this perturbations are periodic, because after one orbit the eccentricity goes back to 0 and the semi-major axis to its initial value.