Pi onoff shim loses power after a few seconds

I’ve just got around to soldering up my on off shim to the provided female header along with some jumper cables to attach one of the mini arcade buttons to the btn connectors on the shim to make a reset switch for it.

i was mightily impressed with myself once I had everything connected and I pressed the tiny power button on the shim and it sprang into life only to have my wonderment turn to despair when at the screen with the 4 raspberries in the boot process the screen slowly faded white from the outside edges inwards and the pi turns itself off.

if i hold the power button down on the shim i can manage to get it to boot all the way up to the welcome to the rasbian desktop splash screen (~16 seconds) but then it simply shuts itself off at that point without fading the screen so I wonder if that’s part of a long press function of the shim instead.

I’m using a rPi3 with the official 7" touchscreen display, official pi power supply, the power splitter sold by pimoroni to power both screen and rPi from one power supply (but have also tried with two separate power supplies for the screen and the pi with the same results).

Could I possibly have a faulty shim? the fact it continues to work longer if the button is held down seems to suggest the soldering to be sound (as it continues to work past the primary issue and could possibly boot fully if the shim ignored the long button press).

is there a log file i can check for the boot process to see if the system is recording anything about the problem before turning off?