I couldn't find anything else reported here, but I found this issue in Nightfall: The Ordeal today having recently unlocked Hush. While my other energy bows (Arsenic Bite, and Subtle Calamity) both work to stagger Unstoppable enemies without too much effort (fully drawn bowstrings, and I tend to be going to precision hits down the sights), I could get this to work with Hush using the same Artifact 'Unstoppable Arrows' mod. A mild frustration, but thought I'd report it here as the linked reddit thread shows there are others with the same problem. Thanks.