two stage system with r422d

can any one explain, why on a two stage system , with r422d the -21ºC coldrooms start raising the temperture once reached -19ºc since the set point for switching of is -21ºc and if the 0ºC coldrooms are switched off. If i switch on the 0ºc coldrooms the tempertures on the -21ºc start lowering very fast and switch off.Once the switch on again they start lowering and when reaching -20 it´s starts raising again. Lack of refrigerant no, meausered the superheat, looked for blocked strainers not find anything wrong. seen the mop pressure on the tvx, this plant used to work with r22, but sombody put in dropin r422d, it works with 700Kgs of refrigerant with Mycom 8 cylinder compressores.