Attached is a simple class and a simple console application. 

if you look at the TPMixed property it is a type String and the private variable that it points to is a String (doesnt' have Type in front of it).  What happens is when you try to set the property the value doesn't hold, in this case a null value is stored (default value for a string) instead of the value passed to the property. 

If you run the attached console application you should see what I mean.