When wired correctly these problems don't exist. So my advice is to check the wiring, but thats not easy if you don't know how the 3port valve works. So first find out how it works.
Now that in itself is not straight forward, but there is plenty of information on it on this forum.
I'll try to explain how the valve operates, which is over two ranges.
In addition to the earth wire yellow/green and neutral wire blue there are 3 other wires White Grey and Orange.
I'd ignore HW operation for the moment and just deal with CH, but you do need to understand the following
Although the cylinder stat which is like a two way switch does play a part if HW is involved. (not satisfied terminal and satisfied terminal)
It should be pointed out that the HW switch on the programmer is also like a two way switch. This means there are two terminals with a wire from each but only one of them can be live at a time.so you have HW ON and HW OFF.
When both CH and HW are not on there is no power coming from the CH ON terminal or the HW ON terminal, but there is power from the HW OFF terminal.
Another thing to remember is the boiler gets its power from the cylinder stat when HW is called for, but if not called for or called for and satisfied, then the cylinder stat can't supply, so the supply comes from the valves 'orange' wire.
OK!. So for CH the programmer supplies the room stat and switched live from room stat supplies the 'white' wire of valve.
The white feeds two micro switches, one deals with driving the motor/quadrant to mid position and on the other micro switch it supplies a PCB, but when the PCB is no longer needed it diverts to the orange wire to supply the boiler.
Having got to mid position the first micro switch cuts off the 'white' but connects to the 'grey'. Now because HW is not involved the grey is live because of a wire from the HW OFF terminal.
So now the grey drives the motor/quadrant to the CH only position and triggers the second micro switch making the orange live.
Point of interest here is that both white and grey are both live, so when room stat is satisfied power to the white is cut off and boiler stops but the valve stays where it is, because the motor is still energised and will stay like that till HW is called for which could be the following day.
You'll realize how important the wire from the HW OFF teriminal is in feeding the 'grey'.
When HW is involved there is also a wire link from the cyinder stat (satisfied terminal) to the 'grey'