Automatic reseting of FTP

I’ve been cycling on Zwift for just over a year, steadily building on my initial FTP score from 170’s to 240’s almost exclusively using workouts in ERG mode. Having entered my first sprint triathlon, I have been cycling around 3 laps of the Watopia Two Bridges course as a ‘free ride’, as it appears to replicate the distance and climb of the event I’ve entered. This has caused Zwift to reset my FTP progressively to 270’s and now into the 280’s. However, when I then try a typical (2 x 20 FTP or over-unders, etc.) one hour workout at this FTP, I’m consistently failing to complete them. So, I don’t believe the reset FTP is accurate and is probably a good 10% too high. Does anyone know why this might be?