Hue Rotation and Color collapsing

I think color balance rgb does not work in HSL.
Switch the histogram to vectorscope, and apply the rotation. You’ll see a kind of rotation there.
Now, whether that makes the tool less usable, or simply different, I don’t know.
According to the documentation:

hue shift

Rotate all colors in the image by an angle over the chromaticity plane, at constant luminance and chroma. You can use this control to remove spilled colored light on a subject or to quickly change the color of some object. This setting is usually best applied locally, using masks.

https://docs.darktable.org/usermanual/3.8/en/module-reference/processing-modules/color-balance-rgb/#master-tab

So maybe it’s more suited to fix subtle colour casts than to actually rotate colours (the suggestion to mask the effect reinforces that, I think).