This is working as designed. The UV channel for the Thread Map can be selected through the Thread UV Mask. By setting one of the values to 1 and all others to 0 it will only use the corresponding UV channel. The UVCombine node used within the graph explains this in more detail:
This node allows the user to select the UV channel with a Vector 4 by typing "1" in the column that corresponds to the channel they want and 0 in the others.
1 in X will use UV channel 0
1 in Y will use UV channel 1
1 in Z will use UV channel 2
1 in W will use UV channel 3