Iridium is very hard so it is unlikely that it just “blew off”. It is also very brittle so it can be easily broken off mechanically, that is why you normally do not re-gap them, it is too easy to break the tip off. There could have been a crack in the tip from the factory or from someone attempting to re-gap the plug, but the tip didn’t completely break off until it was in the engine for awhile.
Today’s ignition systems are very high voltage and high energy. They are capable of jumping a very large gap. When the tip finally broke off, it was likely broken just below the ceramic and the plug was still able to provide a spark for the cylinder.
The reason you are reading infinity across the core is that your meter probe can’t reach the end of the core up inside the ceramic, but the core still exists and a spark can still be made.
Were these plugs the NGK Iridium or the new replacement type you just put in the NGK, or both?