It's always been there, but it's just causing problems.
When you set the sensitivity in-game to i.e. 20, the MouseSensitivity value in the file is set to 20.XXXXXX (instead of exactly 20). This is not the real sensitivity however, but rather converted to the actual sensitivity which in this case is 0.005085 (this conversion is not accurate either, but that's another story).
So what happens if you in your example only change the LastConvertedMouseSensitivity:
(MouseSensitiveName="Scope4X",MouseSensitivity=20.254257,LastConvertedMouseSensitivity=0.001234)
Is that the real sensitivity is now 0.001234, but the in-game slider will still show 20. And if you change any setting in-game, the MouseSensitivity value will be read, converted and overwrite the LastConvertedMouseSensitivity back to 0.005085.
You can skip the whole MouseSensitivity part in the config file, but then all the sliders in-game will show 50.
It's a bit messy.