I use a set resistance and cadence on the bike and it measures avg watts, peak watts, avg mets, peak mets, and they were comparable between the two runs. When I start I go hard the first 90 seconds, 80rpm to get my HR up, then I avg 60-65rpm. I did that in both. The "effort" recorded by the apple watch is a measure of HR, not a measure of my actual effort, the avg watts, etc the bike tells me is a better measure of that.

I was trying to get my HR up but my legs were giving out before my heart was. My breathing effort was similar in both runs, in fact the 142bpm run I felt like my breath was relatively easy.

My HR zones are apple watch, the machine only tells me my HR, (which matches the apple watch).

If you're curious it's a planet fitness stationary bike, manual mode, 16 resistance

It's frustrating when you are pushing hard and your HR just doesn't want to go up.