I see I need to do a lot more learning on FETS and all the specs in the datasheet.
Just for completeness to my question:
I built a lithium powered remote sensor with a NBIOT module. My original design used one of the common 4 pin 3.3V boost modules so I could use a 3V lithium battery to power MCU and then ensure enough power to turn on boost and send data via NBIOT.
Worked well but I could get my quiescent current under 100 uA just doing away with the boost module and finding an NBIOT module that down to 2.1V (SIM7020). Had 2000uA of caps and lots of leakage there but that's another lesson learned.
I had two instances where my cheap NBIOT module from aliexpress ending up leaking current from some SMD caps so I thought using a FET as a switch would give me even lower consumption AND help protect from such a thing happening.
So I read up on FETS and built a little PCB with the same footprint as the boost modules so I could retrofit. I liked the BSP250 because it had low leakage when off but as you can all see, I really have NFI. So when I soldered it up and mated the system to a semi-discharged LI-ion at 3.5V and it just worked, I just made the mistake of assuming it was that easy and all sorted. My plan was 3.6V Lipo4 which is why the battery change from my original 3V lithiums.
Time to do the thing I should have done in the first place and get a better understanding.
Thanks for the replies.