When you change your FOV, you also change the source on which all the ADS and scope sensitivities are based on.
If you have ADS FOV on, the FOV for 3x and below will change based on the hipfire FOV, and this change will exactly match the scaling of MDV 0%, so their 360 distance will stay the same but their FOV will be different.
With ADS FOV off, and for all scopes above 3x, the FOV is fixed, so to keep the same sensitivity as before you'll have to adjust the multiplier (since the source is changed).