Tac x genius resistance dump in free ride mode

The last few rides in free ride mode/race I’m experiencing an extreme resistance drop. Almost always when I put in high power (400-500 watts) on a steep hill does the resistance suddenly drop out and I’m left spinning away. Then the session is essentially ruined because now matter how much gearing I do there’s not enough resistance to get over 100watts. I tried letting my avatar come to a complete stand still and set off slowly but nothing worked. Running Zwift via a brand new iPhone via Bluetooth. Please help as this is ruining my races/free rides. Issue has never occurred when erg mode is activated. Cheers, James