Hello!
Just noticed a trend over time that I thought I could pitch to the forum to ask about. I noticed that when go for a z2 ride, my heart rate tends to trend downward over time the longer I go. When I start my effort, my HR rises normally but as the time ticks up to 1-2 hours, I notice my HR drift is actually downward rather than going up as expected.
Is this simply due to me cycling at a very manageable pace? Should I expect an increase in HR over time even at Z2? Thoughts?
I think that if you are aerobically fit you should not expect to see HR drift over a reasonable (<4hr) Z2 ride. I experience the same downward drift as you, especially inside where cognitive load, bike handling, weather, traffic, etc are a non-issue.
Assuming that’s Z2 power you are riding to? Z2 is a pretty wide range 56% - 75% so are you riding at a constant 70% or is your power dropping? Is this indoors or out? If out then environmental factors play a big part, there may be a slight wind that you don’t really notice but for the later part of the ride it’s actually a tail wind.
Think you are seeing negative aerobic decoupling. I have the same and it’s discussed here on the TR forum and see Google.
I also noticed this during a long SS workout today, tried to screenshot it below. You can see my average HR was lower on the last two 20’ sets. 
A picture of an actual ride would help.
One thing I’ve done to improve analysis of decoupling - ignore the first 30-45 minutes. You really want the analysis done with:
During a long steady state effort I can generate HR swings up/down by changing cadence and changing body position. Summer rides around here can start at 70F / 21C and end at 90F / 32C, that will definitely impact the decoupling analysis and better to not even bother other than for entertainment value.
Also a hard training block, or even 3 hard days of riding can impact your HR and that can impact decoupling.
I also have the same thing sometimes, I just claim it as a long warmup.
For my self on a steady state Z2 ride I would expect my HR to be pretty flat and not drift much and perhaps even drift down over time once I’m settled down and moving efficiently. As the intensity goes up though I’ll see some larger drifts upwards.
That’s quite usual for me when the power goes down slightly as it is in your workout. Nice power figures btw 