IMO, good preventative maintenance, and you were able to do it yourself - good for you!
You could not have known their condition otherwise.
What time s the total mileage on the vehicle?
Many years ago, I had 2 issues ( different cars, small-block Chevy, and VW Rabbit) in which the internal resistor messed up somehow. Plugs were not that old as I recall, but combustion side turned black from occasional missing. I could read greatly different resistance on the bad ones. Changed the plugs and restored perfect operation. Years later, changed those plugs and they were all perfect light brown.
Similar experience on common Briggs & Stratton lawnmower - changing last year’s perfect-looking plug with correct gap changes a non-start into first-time start. One variable!