Resistance on trainer inconsistent

Specs;
Windows 10
ANT +
Kickr Core Trainer
4iiii Power Meter (used in Zwift as Power Source)

Not sure if this i Kickr problem or that it has something to do with Zwift.
But when i do rides the resistance can suddenly become really strong on a -3% or drops on a
6% gradient.
This happens on various occasions during a ride…

When trying the Jon’s short mix on erg mode it was impossible to do.
first interval i got up to 350w and the restistance dropped making it impossible to hit the wattage to complete the workout. I would have had to have a cadence of 200 to make that happen.

Does anyone know what the issue can be ? is it some kind of interverence or a software issue ?

Or can it be because i use my power meter as power source and the trainer as the main source ?
would be a shame as my power meter gives more precise readings…

any help is appreciated.

This could be the case in ERG mode.

also look at this, https://zwiftinsider.com/erg-mode-in-zwift/