I use a very simple set of rules to turn front porch lights on and off based on sunset and sunrise. No issue for the past few weeks. Yesterday AM the electricity momentarily went out and came back- rebooting all my systems. In the evening the lights did not come on. I ran the rule manually and this morning the lights did not switch off.
Sunset happened- but the rule did not trigger. Sunrise never happened.
Check your system time. The HE hub doesnt include a real-time clock, so after a reboot, if internet is not immediately available for NTP, then the clock stays un-set. I would also recommend installing the NTP client driver.
I checked the settings-> hub details- it has a current time. 
I see you mention that if internet is not available immediately- that might be a problem. so I did a reboot of hubitat to simulate power loss without losing internet. However- no luck. no scheduled events still.
I finally had to do an "update rule" to get a scheduled event setup
I will try with the NTP client driver though. Not sure if this is my ignorance in how rule machine works- but doesn't hubitat periodically poll the different rule machines and see if anything needs to be done?
Mike responded in another post a while back that if you use offsets like + or - x minutes from Sunset or Sunrise then you will have this issue for 24hours after a reboot, the next day will be fine again. If however they are set to occur exactly at Sunset or Sunrise they should occur. This was my experience the last time I restored my DB backup which causes the same behavior.
