Rick and I were just talking about this... and the problem seems like it stems from system_event_t being 64-bit as you pointed out @Ric Casting to int fixes it for now, but later when we have more events up in the upper 32-bit range you'll need to shift them down and break up the event into two 32-bit unsigned ints.
// %llu is uint64_t type, but does NOT work
Log.info("EVENT: %llu DATA: %d", event, data);
// Good workaround
Log.info("EVENT_H: %lu EVENT_L: %lu DATA: %d", (uint32_t)(event>>32), (uint32_t)event, data);
The reason this doesn't work is because it's not supported in Wiring, unsigned long is the highest size supported: