I'm not sure what to tell you... I've never seen this before. I'm using 16.81 on Ventura 13.6.3 & cannot reproduce the split swatch.
Have you tested other color tools & in other programs to see whether they produce the same oddity?
All I can suggest is to see whether restarting the Mac provides a remedy. If not, use Help> Feedback in the Excel menu to report it to the development team. Include a link to this thread when you do. If you witness the same issue in other programs it should be reported to Apple as well.
The border around the Color Wheel tool indicates that the tool is selected. If you press Tab repeatedly you'll see how the selection border moves from one element to another. However It doesn't seem to enable activating anything but the OK button.