Speed not reflecting power

Hi, looking for some guidance.
Last couple of rides I have noticed my power output is not reflected correctly in my speed. For example, this evening climbing Alp D’Zwift I was climbing at 250W averaging around 8-9mph. After I was done descending and back to 0% gradients I was at 250W at still at 8-9mph, having to generate ridiculous watts to get to a decent pace.
Trainer etc calibrated before every ride, only started experiencing this.