Apple Watch fitness

I recently purchased an Apple Watch series 6. I completed an hour workout. For example I started with 20 minutes on the stairmaster machine. I did 20 minutes on a high level speed I was clearly working hard as I was sweating immensely. The gym equipment was showing I burned 260 calories. I have cause for concern because my watch only registered that I worked out for 8 minutes and burned 130 calories which I think is wrong. My heart rate was clearly high for the full 20 minutes so I’m unsure if the Apple Watch fitness app is reliable but this is pretty disappointing as I made this purchase strongly for fitness.