Resistance varying going up hills - Tacx Flux

Continuing the discussion from TACX FLUX S - dynamic resistance going up hills (resistance comes and goes):

I have an issue - iPhone Bluetooth to Tacx Flux, just upgraded from Vortex Smart. Fine on the flat but as soon as go up a sustained hill (eg Box Hill on Zwift) the power and resistance is all over the place - +/30% in a matter of split seconds and this lasts until the hill finishes. Any suggestions welcomed - it’s making proper training v difficult.