Tacx flow smart wattage incorrect on gradients


I have been using a tacx flow smart for a while now. I also have a power meter so I can compare the watts in zwift and on the bike itself. I have been noticing lately that the watts are pretty accurate if the gradient remains 0%. However once the gradient goes up I notice a difference in watts of around 10%. This is pretty annoying as I don’t want to cheat.
I calibrate the trainer every ride and same for my PM. I have tried to put more tension on the wheel but this is not the solution.
Anyone have a clue or a tip how to get this better?

Thanks in advance!