Shower mixer temp. adjustments really wonky

Our showr mixer is really hard to change the temperature on, it goes from cold to hot in the smallest possible increment. The manufacturer came out and changed it and the thought it was due to debri in the inlets screens but a couple of mon5lster the exact problem is back.

We have decent water pressure and before the invented cylinder there is a pressure reducer, might that be the problem? Nu if so, can I test that hypothesis somehow?

The mixer is a Aqualise Midas 110.