I reported this bug about 5 years ago and it still exists. The feature "Add Swatch to Color Set" in the mixer panel options is broken. It does not work, period.
To reproduce it, just go to the help entry at this link, and follow the instructions under "To add a Mixer color swatch to a color set":
I realize this isn't the biggest deal, and there are simple workarounds, but come on, it's just sloppy. This has been reported and filed as a bug, it keeps carrying through in the documentation, and yet it has been 100% broken since at least Painter 2016.
You are right, I just tried this - doesn't work at all and needs to be fixed.