Created attachment 103620 [details] Hue bar, when saturation is low 1. When Saturation/Value is high, hue bar is visible, but when saturation/Value is low, hue bar is not visible. Find four images here: http://imgur.com/a/s3iSj
This does not look like a bug, but intended. All three little arrow (one for each slider) should indicate the exact same RGB color.
Where can I send user interface suggestions and ask queries?
You should start on the forum: http://forums.kde.org/krita and when there's a consensus about a way forward, take the idea to phabricator, where you can work together with UX designers and developers (when they have time). As Christoph said, this widget works as intended.