Created attachment 181390 [details]
sample picture, "color balance" test, preserve luminosity selected

The GIMP manual says regarding filter "color balance":

Select range to adjust

Selecting one of these options will restrict the range of colors which are changed with the sliders or input boxes for Shadows (darkest pixels), Midtones (medium pixels) and Highlights (brightest pixels).

However, selecting "shadows", moving the slides has little impact on the dark pixels, but changes the bright pixels in a drastic way. This behavior, however, is not expected with the selection of "shadows".

I added two samle picture. They differ in the selection of "preserve luminosity". There are 4 layers. The top layer (layer A) is the origin, a white to black gradient blend. The three layers below have been subject to the appliance of "color balance", with "highlights", "midtones" and "shadows" selected respectively.

As you can see, the result looks quite different from what would be expected. Especially the "highlights" and "shadows" layers look quite the same.