RC low pass filter not working as expected
You're not showing the stray capacitance in parallel with R310, R318 and R319.
In a voltage divider, the C is often proportional to the R so no harm done, but in the filter, at 36 kHz, it could easily dominate the 1 M resistor. Each 4 pF of stray capacitance there has an impedance of about 1 Mohm, so if the nodes HV2A and HV2B are fairly bulky this could potentially be a problem.
The solution would be to reduce the capacitance, or to reduce all the filter component values to reduce the impact of the stray capacitance.
No simple PCB-mounted filter can achieve -120 dB though, for that you will need to separate the filter elements into their own milled aluminium housings. The PCB ground plane resistance and the inductive coupling between C322, C326 and C327 if they're nearby, will limit you to -60 dB or so.