The coolant is building up in the cavity/hole the spark plug goes through and into the head? There's not too many ways for that to happen. If something like the turbo coolant lines or another pressurized coolant hose were leaking and spraying underhood you
should able to tell fairly easy.
With that said, I believe there are higher than normal failure rate on the
coolant outlet/tube/fitting near the driver side of the head. It's under the engine cover and could possibly be spraying coolant up onto the valve cover.
Other than that about the only other way for coolant to be building up in the plug hole (assuming you're sure it's coolant) would be for a cracked head. Sometimes very slight cracks will only weep when hot and the aluminum expands and under greater coolant pressure and not show when pressure testing a cold engine.
I don't recall hearing about problems with bad castings on the 1.4L so I'd very carefully check any coolant carrying fittings and hoses first. If it is the common plastic coolant outlet the good news is it should be pretty easy and fairly cheap to fix.