I sure would like to know what exactly is going on with this haha. The hue and saturation gain a mind of their own when I adjust the value slider in the Theme 3D Viewport Gradient color section.
That's a bug! Report it! It only happens when using the slider on the right hand side. When using the actual Value slider, the Hue and Saturation stay the same.
I reported it. I was able to confirm it is still doing it in version 2.9 alpha
https://developer.blender.org/T75876
I noticed that it even does it on the HSL 'Value" Slider. it is just not as pronounced and seems to 'bounce back' better. I tried uploading a blend file but I don't see it on the posting......I edited the post to add a reference to this forum posting. Does that remove the file when it gets edited?