When enabling the combat text on a unit frame (e.g. Player or Target), a reload of the UI is required to make it work.

I did some debugging and this may be a bug in FrameXML code, so perhaps best course of action is to add a note in the config UI of LUF?

After the line above is executed, element.feedbackText:IsVisible() is supposed to be true because the last line of CombatFeedback_OnCombatEvent is feedbackText:Show();.
However, element.feedbackText:IsVisible() is consistently false until you perform a reload of the UI.