Use of App across all iOS devices
I've been debating purchasing an app for my iPad Pro to allow me to edit my websites database while I'm out and about. While I was in a cafe the other day I needed to make a quick update to an entry, so I decided to purchase the app using my iPhone (I didn't have my iPad Pro on me at the time) as I was under the impression that it will automatically transfer over to my iPad Pro.
Much to my annoyance when I tried to use the app on my iPad Pro it requested me to make the payment again. So as it stands, as I have an iPhone, 11" iPad Pro and 12.9" iPad Pro I'm expected to pay 3 times for the same app. Am I right in assuming that if you purchase an app from the App Store, that same app can be used on all iOS devices as long as they are signed in using the same Apple ID?
I have requested a refund from Apple, but I have been declined the request with no reason to why.
Thanks in advance.
iPhone 12 Pro Max