I'm playing on a Hisense Gaming TV that supports 144hz and has this built in fps counter. You can see how the fps actually fluctuates and I'd say the average is more like 100-105, not locked in 120 fps.
For reference the game/s never had such fluctuations on the 60fps mode (or on the PS4) where it's almost locked to that framerate with almost no drops.
It's definitely not an issue, just decided to point that out. Also another interesting thing that many people probably don't know is that the game quality differs between 120fps setting or not. If you turn it off you'd get noticeable better graphics and shaders, and vice versa – this setting turned on means a small degrade in the graphics quality.
So it's more like a Performance vs Quality mode, akin to other console titles (GTA, Spiderman etc..)