This is not a bug, it is intentional. When the application is executed on small monitors with horizontal resolutions under 1920 logical pixels, this icon is not shown. This is to favor inclusion of more important (IMO) icons, such as AutoStretch icons, in the default toolbar configuration.
If there is major consensus in that this toolbar button is so important, I can rebuild the ColorSpaces module to bring it back at all resolutions. Anyway, you can always extract lightness with IMAGE > Extract > Lightness (CIE L*), or directly with the ChannelExtraction tool.