So do you calculate your heart rate zones based on your max heart rate or from your FTP test? Does your heart rate get up into zone 3 pretty quickly on a zone 2 effort or is this after a couple of hours of riding? Also having a different indoor to outdoor FTP is not uncommon. Did you have a one time 20 minute effort that was far greater than any other effort you have had that maybe inflating your FTP? Do you have similar efforts? Do you feel like you could hold that FTP for an hour?
The other thing you would want to look into is if you take in caffeine before or during your working out which would increase your heart rate. Also since we have been in summer with the heat and humidity riders heart rate tends to run higher.
Should you follow power or heart rate depends. You should also look at perceived exertion. If it feels like zone 2 most of the time you are good. You will have some heart rate deflection as the duration progresses. Ideally through training this deflection point moves further into a ride. If you have a hard training day following you will want to monitor your heart rate and not push the effort too much adding more fatigue. In our Sweet Spot 4 plan that is why we put zone 2 by heart rate. That way we can keep athletes from pushing it too hard and putting too much fatigue on the body for the harder efforts.