I'm on 14.4 12Pro Max, and it still happens. It's been happening since earlier OS versions and iPhone X for me. I'm not sure what affects it either, is it iOS, or ... it's all normal, and then I open YouTube, Imgur or any other app with some sort of volume control and then notifications volume "goes up". Reset, restored, gave it a mean stare, tossed against the wall ... nothing seems to help.