PTC tracking computation issue when momentum spread is included

[LucaBiasia]

Hi, I am trying to track 1000 non-relativistic muons (28 MeV) passing through a quadrupole using MAD-X PTC library. In the case of monochromatic particles I get the expected results (compared with other programs). When I try to include momentum spread in the sampled particles (I sample only the particles on the border of the phase space ellipse) I would expect only a slightly deviation of the points from the phase space ellipse border, but I get the artifatc you can see in the following image (representing the normalized final phase space): This gets worse if the momentum spread increases and also if the CORRELATION in the initial phase space increases! The following plot shows the same result, but with -0.995 x axis correlation and 0.995 y axis correlation:

I tried to apply different models for the PTC tracking procedure but i didn't manage to find the reason behind this problem. I will attach an example of the python code below. particle_tracking.zip

This is how the initial phase space looks like:

Thank you for your help!