Have a high performance motorcycle engine that doesn't have an oxygen sensor which means the ECU has no ability to tweak air/fuel on the fly. For the first 40k, completely stock it had no issues. Gradually it started to develop cold-engine run/start issues. Later began fouling the plugs/cylinders. I was able to compensate initially with additional (Lucas brand) octane; later blocking off up to 50% of the radiator; later hotter plugs. Progression of the symptoms were gradual. 15k later, what actually resolved the issue, (allowing an undo of previous changes,) was telling the ECU to enrich the fuel supply.
Obviously we're running a wideband O² sensor and on a stock tune, there's no reason to suspect that you're running lean. The non-stock colder plug is the logical assumption. The manufacturer seems to run their direct-injection engines a lot hotter than previous port-injection; I suspect for that very reason -- keep them clean. Typically one needs to go colder in order to help prevent pre-ignition. Higher compression, hotter engine and/or advanced timing are conditions that might warrant a colder plug.