New to Zwift, would welcome advice on HR Zones

Hi all, I’m in my first month paid subscription and have signed up to FTP Builder Training Plan. I should mention I am male, aged 66 and 80 kg. My setup is just a wheel-on roller (not ‘smart’) and I use CooSpo HR, speed and cadence sensors, so no power meter. I set a FTP of 120 as a starting point (1.5 x 80kg - is this reasonable?) and am now into the 3rd week of the plan.

The previous workouts I haven’t really paid much attention to my heart rate but the latest Foundation workout was suggesting that it was aimed to take place in Zone 2 (which should be Fat Burn, right? and for me should be 93-107 bpm). However, even in the 65 Watt warm-up my HR got to 111 and when the 6 minute 85 Watt efforts came along it increased throughout until it was 159 bpm at the end. The 1 minute 65 Watt rests in between didn’t really slow it down much at all. The 10 minute cooldown at the end saw it only reduce to 144. I should stress that I managed to maintain the required Watts and cadence (just) and wasn’t out of breath or in pain, but can this be right? Is my physiology not set up correctly or my trainer?

Here is a screenshot of the effort:

Hope to receive some helpful feedback!

Stay well people.