There are two things to keep in mind here:
1. Whenever Final Cut starts acting weird, it is time to trash FCP preferences. Preference Manager is a good option, you can also do this manually, which I’ve written about here: https://www.larryjordan.biz/trouble
2. The single-wheel color corrector is OK, but designed for business graphics. Most of the time, you have more control using the 3-way Color Corrector filter. You get the same level of gray-scale control, but much more control over color adjustments.
Larry
https://www.larryjordan.biz