Please see comments
Historically, the kstats such as cpu_[user|system|idle]_ticks would account for time spent handling interrupts by assigning such tie to cpu_system_ticks. In Solaris 10, an unintentional impact of microstate accounting is that it does not specifically assign interrupt time to any category. If the CPU is idle when the interrupt arrives, interrupt time goes to cpu_idle_ticks. Same for cpu_user_ticks.
Clearly this behavior is different from S9, and thus this is a compatibility bug. It's also clearly not a useful way to provide statistics. This bug intends to restore S9 behavior.