I think it may be intentional. It seems that if the alarm is more than 21 hours away, it will not show on the lock screen. For example, right now it's 10:30am as I type this. If I set an alarm for 7:30am tomorrow, it will show on the lock screen. But if I set it for any time later, it will not show up for a while. If I check three or more hours later, the alarm time will finally show on the lock screen.
This seems intentional and frankly really annoying. I feel like if I set my alarm, it should show on the lock screen no matter what time it is today. But for the most part, as long as I check my screen before bed, I know it will be there and can be assured that I did set an alarm for the morning.