Well I’m not sure what sort of sensors they tend to use in your area, but the sensors here aren’t looking for your ass or weight on the toilet. Here they use a wall mounted sensor that apparently tries to detect your back and I guess the distance of your back to the wall.

If you’re not sitting almost perfectly upright (as in leaning forward to get some toilet paper), the stupid thing thinks you done got up and triggers the auto-flush.

Not like I expect anyone in the comments to be any sort of plumbing/sensor expert, but the systems and sensors around here are usually made by AquaVantage/Zurn, if that makes any bit of difference.