Something I’ve noticed lately is that my heart rate is usually in a zone below what my power zone is during workouts.
For example during a 15 min interval I’ll be at
Zone 3 sweet spot power
Zone 2 Heart Rate Avg
Is that pretty common?
Yes, especially if it’s a short ride. If you ride hard for 4-5 hours you’ll sometimes find the opposite, where your heart rate could be zone 3 when riding zone 2 power by the end. They don’t always line up exactly.
I had that issue when I was using Friel’s HR zones and Coggan’s power zones. Switched HR zones to correspond with the Coggan zone % of LTHR, and issue was gone.
When that happened to me, the cause was long-term fatigue, something that @kurt.braeckel pointed me towards. I had erroneously thought it was fitness.
I’m not saying this has to be what is happening here, but it is a possibility.
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As others mentioned, this can be quite common – HR can vary due to a lot of factors (such as how much sleep you had last night, how much coffee you’ve had today… All sorts of stuff!).
We generally recommend looking mostly at your power data for that reason. Anomalies, such as seeing your HR way higher/lower than it normally seems to be, could be a sign that something is up… But if your HR stays pretty consistent most of the time, it shouldn’t be anything to worry about. 
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Do you monitor your HRV (Heart Rate Variability?