I have at least 10-15% power “bonus” on hills compared to flats or workout mode. Some stats: in workout mode I can manage about 210-215 FTP with a big effort. On Alpe du Zwift I can easily hold over 230 for the whole hour, and today I got an FTP score of 233 from my last 20 minutes when I ramped up the effort, still had something left at the end. My slider is set to 50% but that shouldn’t influence how long I can hold a specific power level. There must be some issue about how the power readings are processed. I’ve seen other posts with similar although less pronounced effects, although some of them have reported the opposite effect, i.e. lower power on hills.
I have a Tacx Flow Smart which I calibrate every time but that only ensures the wheel tension is within range, doesn’t influence power measurement.
I would bet this affects many more people. It would be good to resolve it, I’d quite like to know my real FTP!