He could have approached the valve with some rubbish (I also had it in the bathroom once and I have the same valves). You can remove the head, temporarily set the nub (maximum) to the increased flow to flush the valve, then restore the previous initial setting (cross banding) and install the head. There may also be another reason for this, namely a damaged thermostatic head, e.g. a broken metal bellows with gas in the head (as the temperature increases, the gas expands and pushes the plug which presses the needle and closes the valve, and when the temperature drops the gas contracts, and the plug retracts, reducing pressure on the needle). A head that is damaged in this way (pierced bellows) can only be replaced. You can check the head by observing the mushroom's reaction to temperature in it. For example (we do it on the removed head) we measure the distance from the plug to the edge of the nut. Then we put the head into the freezer and measure this distance again. After such a strong cooling (temperature in the freezer around -18 to -20 degrees, sometimes even colder) the difference should be significant.