So you basically you get ,Quest Completed" feedback after every 50 stacks.
Most heroes with modular quests ex: Nazeebos Thing of the Deep don’t provide such feedback. It is probably an artifact from before recent Valla rework where she gained bonus only once.
A video that shows both Valla get a feedback from her quest and Nazeebo who doesn’t get the feedback. Both provide similar value thus I can’t understand the inconsistency.