Well, I had to try it…
Interesting: some apps like Audible which have settings to only download while on WiFi really stick to just that - they won’t download if the phone is connected to Ethernet. Others like Downcast are fine with it, though they have the same kind of WiFi-only preference.
I assume some are checking “not cellular” and some are checking “on WiFi” perhaps? I know nothing about iOS apps, curious though!