PMU Bec need at least 10V to work.
If you use 3S battery, you may need to set up low voltage protection, or replace 3s battery with 4s battery.
https://discuss.ardupilot.org/t/cuav-core-x7-and-nora-experience/60073/12
I just checked my CUAV HV PM that comes with a V5 Nano. It doesn’t suffer the same issue.
The 5vdc output is steady 5.26v at the FC over a wide range of input voltages (even with a RFD900 running off the telem port) and it drops to 0volts when the battery voltage goes below 8.18v.
I think that’s fairly acceptable for 3 cell Lipo usage.
Note: the HV PM looks the same but it’s got the older style analog voltage and current sense wires, not CAN