It's not a bug, but a design choice. The Amount knob is used some places in Settings, e.g. for the MIDI channels. So for consistency it does not adjust mod slot amounts when the Setting menu is active since it's also used there. Likewise, when the Ribbon or Morphee page is shown, that takes precedence over the mod amount display, and so the amount can't be changed "blindly".
I agree that in the latter scenario. the mod Amount display could override the Ribbon and Morphee pages, but it would cause a bit of display 'chaos'. If you want this changed, you can submit a support ticket with it as a feature/change suggestion.