So Bungie... I feel like this oversight was forgotten as Iron Banner phased out of rotation. With it being back this week, wanted to bring this issue back to light. The Wicked Overgrowth shader appears as unacquired in collections for those that completed the seasonal Iron Banner questline on all three characters prior to the September 3 "fix". However it is accessible and can be applied in gear customization menus regardless. There does not appear to be any way to now reacquire the shader to trick collections into updating it being collected...