Problems with Zwift Hub resistance, using with IOS

My IOS, Hub and Zwift app are all on the latest versions, zwift hub is only a few weeks old.

I had the difficulty of the trainer on the default of 50% and had a issue when climbing uphill (not massive gradients, 3-5%) where resistance would drop to zero and cycling at 120 cadence resulted in 1-2 mph speed, this is on the smallest cog of the supplied 11 speed cassette (highest gear)

I’ve increased the difficulty to 100% and the problem is better, but not disappeared. Also when cycling at 0% gradient the resistance again drops very low and then after 10 seconds or so the resistance kicks in massively, even though the gradient has not changed.

Has anyone any advice how I can resolve?

Thank you!