Hello ,
I had exactly the same problem IOMCU reset on an autopilot not used too much pixhawk 4 holybro,firmware( custom) 4.1 stable,the flight was low altitude controlled and protected for test, it was like the drone lost control for half a second after it took control again but I cut the motors to avoid taking any risk.
but when i saw the log i was surprise that rcouts were frozen for almost 2 seconds !, then after that weird radio failsafe and land, I asked this question in this link RCOUT freezes during the flight, IOMCU reset! no answer yet ,there is the log , but i need to find the cause because it can be dongerous if happen again , ,why the IOMCU reset !
I must specify that the firmware is custom because I adapt it to my Heli coaxial drone, then I use six servos and two esc, I believe that this IOMCU problem is caused by some kind of electrical disturbance between the outputs and the escs servos, is it possible that the pixhawk hardware is poorly electrically isolated!?