Heart rate has many variables so we try to stick to power. The first major factor is fatigue. If doing a longer endurance ride your heart rate will increase over time. To properly train the system you want to maintain power even if heart rate goes up. Over time you would like to see this deflection happen much later and be a lower percentage.
Another factor is heat! We are in the middle of summer, maybe back end of it, but it’s still very warm in many places. The heat can make your heart rate higher. So again best to stick to your power zones. The zones are wide so you can ride at the lower end.
There are other factors such as rest, sleep, caffeine and etc that can lead to variables in your heart rate. The great thing about power is it is accurate, instant and does not have these variables. When was the last time you did a power test? You may want to complete another one so you are working with an accurate power FTP and heart rate FTP.
Still listen to your perceived exertion as well though! It’s a three way system, power, heart rate and perceived exertion. I really like to have athletes feel at least two of those are good .