From what I’ve tested over here… I think the latest iPhone firmware (on my iPhone 14 at least) does something to detect the power draw request from the USB device over the iPhone camera kit (then USB -> USB-C) and basically just says “Nope”. I have tried using the powered camera kit as well and it does the same thing. Pretty frustrating as this is a basic behavior change for the iPhone and camera kit, for something that “just worked” before.
At least on the P-6 we can tell it to not draw power, but the SP404 mk2 as far as I know doesn’t have a way to do this.