Yes, it is strange to have FICVF to be uniformly 1. Are you scanning your sample ex vivo? If so, have you used the corresponding model 'WatsonSHStickTortIsoVIsoDot_B0' instead, as described previously here?
I should also note that your assumption about the fractions adding to 1 is NOT
consistent with how the model has been constructed, and described, in my paper. In fact, FISO and FICVF are two independent variables, each of which can vary from 0 to 1. Take a look at Figure 2 of the following paper: