I am hitting the same issue now. My guess based on the power meter is similar - the Anova WIfi uses coarse PWM to essentially flick itself on and off. Even in the steady state (covered pot, target temperature reached).
I use high voltage incandescent halogen lights, and they flicker quite a bit. All other appliances are in standby mode, and the circuit easily powers much, much higher maximum wattage easily enough.
From my experience with building my own PWM bits (aren't arduinos fun!), I'd suggest Anova considers to either use a much higher frequency for the PWM (so as to render the flickering invisible), regulates down the current for the stable state (does it have to flick the relay at the full 900W? really?), or indeed does just that - instead of using PWM, regulate the wattage and current going through. The later would likely be cleanest (similar to PWM LEDs vs the newer ones), but also unlikely to be possible with a firmware update.
I wonder if it's just the EU Wi-Fi version that exhibits this?