besides enabling 4 additional channels, the changes included:
introduce new UART message between PX4 and M0065 board to allow specifying output pulse with high resolution (previously limited to only command range of 0-800 which was remapped to 1-2ms) change the output pulse to OneShot, which means for every command from Flight Controller, there is only one PWM pulse (per channel) generated. This synchronizes the PWM output to Flight Controller update cycle and reduces total delay between FC and ESCs. there was a bug in case of SBUS input was used, the output PWM rate was reduced to only 100hz (normally about 400hz using 1-2ms pulse, which is a limitation of the pulse width, since flight controller update loop is 800hz).I am curious if you were using SBUS input into M0065 before for your RC, in which case the pwm update rate was reduce to only 100hz.
Change to OneShot should not have effect on the pulse width, only the delay of it. It is possible that with new firmware, the ESCs are getting updates at higher rate, which could mean that if there is high frequency noise (picked up by gyro from propeller vibrations), the ESCs are now receiving more noisy commands, which would cause the motors to heat up more.
If you have the PX4 logs from before the M0065 firmware change, you can take a look and see if gyro was picking up a lot of noise (more than 5-10 deg/s is considered more than normal). Additionally you can check the noise in the actuator outputs.
Are you still using m500 frame? did you modify it in any way? maybe VOXL2 needs more vibration insulation. Also, what you could do is restore the original M0065 firmware, use unmodified PX4 and compare results again (making sure to save the logs).
Also, did you go through the ESC calibration and updated the PX4 actuator ranges, as instructed, after uploading the new M0065 firmware?
Alex