Great work with the pacing and your race. You nailed it!
In any endurance race heart rate becomes depressed. This is normal and happens to everybody. How soon it happens depends on how fast you are riding but most commonly it starts to kick in at the 2.5 hour mark if you are fueling and hydrating correctly. If you had a power meter you would see your HR drop off and power remain stable. Watching your lap splits and speed confirmed your power did not drop off and pacing was perfect.
Pace the same way in the next race! You nailed it. Effort should increase in the second half of the race, power and speed should remain stable and HR will remain stable or drop due to HR depression.
If your HR rises in the second half of a race it is most likely that you are becoming dehydrated and your speed will subsequently slow.
Good work!
Lynda