GIMP-2.99 doesn't show small negative numbers in FG/BG sliders
Open this XCF file and make sure the top layer is hidden and the color picker is set to show the merged color and to change the foreground color, and to use the Info Window. Also open the Change Foreground dialog:
Pick the various colors. Any channel that is slightly negative - as easily happens from doing precision conversions and/or ICC profile conversions - fails to show a number in the Change Foreground sliders. Here's a screenshot:
Now make the top layer visible. This layer is the same as the bottom layer, except I used "Exposure" to make the "near zero" channels be considerably less than zero. Color-picking from this layer, all the negative channel values in all the sliders have actual values showing, eg -39.5%.
This doesn't happen on GIMP-2.10.