I've often said in my Zenitsu meta that a lot of his greatest moments come without witnesses.
Given how motivated he was to get praise and admiration--well, not so much that but being seen as more useful to others than he otherwise ever was--was a big part of what inspired him to stick with the swordsmanship path that he otherwise hated, this lack of recognition instead adds more weight to his development.
True to his final words exchanged with Kaigaku, Zenitsu knows right from wrong, and he has chosen the righteous path despite its thanklessness and hardships. He chose not to share knowledge of this choice, because at that point, the recognition for doing good was not of value to him--doing the right thing was worthwhile even if he kept it to himself.
I've seen both humor and speculation about the lack of Zenitsu's solo triumph over Upper Moon Six being shared with the others throughout the Infinity Castle, but I've generally always been fine with it. It fits fine with Zenitsu's arc. Plus, Zenitsu got all he needed from just a few words of affirmation from Kuwajima.
Sure, as a fan artist and fanfic writer I have loved the idea of somebody being there to witness Zenitsu's feats and accomplishments, and I also love the angst potential of his secretiveness. But it seems someone at Ufotable has always wanted Zenitsu to be recognized too, and that's why we have the Kakushi talking so extensively about him when they are on the scene at Mt. Natagumo.
But also, with the addition of the Kakushi in this movie, I would love to think that this happened after Zenitsu's victory:
It would have been such a great noise for him to hear.
