Just recently one of my car keys has stopped turning off the immobiliser on my 2019 Plus 4 GDI.

Starting procedure is "touch end of key to the ignition key block" (I mean the plastic end of the key where the RFID chip is)
Immobiliser light goes out - insert key - start engine.

Did this several times the other day - and immobiliser light didn't go out - therefore non starting car

Got my spare ignition key - procedure as above - immobiliser light goes out - car starts no problem

From reading the forum and my simple understanding, the RFID chip in the key is "inert" - There is no battery in the key (basically like the security tags on whisky bottles / clothes etc in shops).
So in "theory" - nothing can go wrong with the chip?

Thoughts?

The chip is inert. No battery. But sensitive to temperature, and super sensitive to distance from the sensor ring.

In theory both keys should respond the same, although 1mm difference in location in the key would be enough to make that difference.

Remove the cowl, and check that the sensor ring is as far out towards the key as possible.

As Dave says that is a well known issue.

I physically touch the red section of the key (pic above) to the ignition barrel - so no problem with distance

The RFID is a passive transponder usually embedded in the key but can be standalone as with M3W. The car immobiliser transceiver in the steering column sends out pulses of a very weak RF signal at 13.56 MHz which the transponder in the key bounces back modified with its unique code. The transceiver gates out any incorrect code but allows correct to disable immobiliser. Being a very weak therefore range sensitive signal added to the anomalies of RF transmission there are many factors contributing to it not functioning correctly.

Would the RFI chip in they keyfob of a 2019 car be truly inert ? I understand it does not need a battery as it derives its power from the incoming RF signal, but would it perhaps have an EEPROM to store the particular code for that car and some digital processing circuits. So there would be active components (in the transistor sense) that could fail, rendering the key inoperative. Though it does seem that super sensitivity to key proximity is the main fault mechanism for the immobiliser not being unlocked.

Does anyone know exactly which chip is used in the keyfob ? Just curious (as ever....)

This is one version of a car key RFID system:

https://cecas.clemson.edu/cvel/auto/AuE835_Projects_2011/Velankar_project.html

It may be possible for the capacitor to lose its ability to hold charge.

Ian's key is not the same as my 2016 key. No red block on mine, the transponder is in a drilling at the key end, and covered over with a sort of epoxy filler.

Not inert but maybe passive Dave in the sense it uses none of its own energy. The RF energy comes from the car. Probably simpler than you imagine although likely a programmable chip.

FWIW just found another source which suggest transceiver frequency is 433.92 MHz in Europe so bottom end of UHF band.

On my 2018 key I took the pill out,turned it through 180deg and moved the working end beyond the body of the key and stuck it on the outside of the key with silicone. Then put some heat shrink around it.
Just put the key in the barrel and it works first time 99% of the time.
Edit. PS, the heat shrink does need replacing every year to 18 months.