ESP32: Power rail transient when using SPI on battery
Thanks for all the advice you guys have given, it turns out @Cisco25 was right and the PMOS was starving the 3.3V reg. When shorted the battery straight into the regulator on our PCB, it did remove all of these artefacts. (don't know why it didn't work the first time I tried it, but there ya go)

We also added a 1nF cap at the input of the reg and a bypass cap on the 100k resistor which seemed to help (thouh when we shorted the PMOS this was made redudant).