Many of these are very sensitive to how they are plumbed..what kind of pipe is used, and if the proper outlet is used. While some valves may have two outlets, one going up for the shower and another going down for the spout, some designs won't work right if you use both. Gravity often wins when water is concerned, but if there are any restrictions, the water can back up and fill the riser tube for the showerhead. You'd have to read the instructions on that particular valve to be sure, but they may have taken the plug out of the second outlet to make plumbing easier, when the manufacturer has indicated you cannot do that or you get exactly what you are seeing. It can also happen if you used pex to feed the spout, since it is smaller inside than copper, and restricts the flow. When the valve is set to warm, you are getting a good flow from both the hot and cold - the max available which is why you are seeing that. When it is only flowing from one side or the other, you have less volume available.