I always squirt MAF sensor cleaner over a clean piece of tissue. That way you can see how much muck comes off Tim, although it's always been spotless. Maybe your checks just disturbed something that was amiss or it was a plug gap issue.
Richard
2018 Roadster 3.7 1966 Land Rover S2a 88 2024 Royal Enfield Guerrilla 450 1945 Guzzi Airone