I’m late to this, but thought I’d comment. You mentioned a Tempo workout. There can be quite a range of power levels for a Tempo workout, 75 to 90% of FTP. At 90% of FTP that should feel quite hard. If you attempted a workout at the high end of that range and the intervals were long / the recovery times were short, that might just feel too hard. A well constructed training program should walk you up to a higher percentage of FTP, longer intervals and shorter rests between intervals over a period of time. If you selected a Tempo workout from the large list of Zwift workouts, it might just have been that you aren’t ready for that yet as opposed to your FTP not being properly assessed.