Since iOS 13 (iPadOS) Apple has made changes to the default browser (Safari) so that it now behaves like a desktop browser (see here for details, under the Safari section).
This means that links sent to an iPad will not prompt the user to download the App. We are looking at fixing this behavior, but in the meantime the workaround is to ask your users to follow the procedure below in order to request the Mobile version of the Website (or ask them to download the app before clicking on the link sent to them).
This is the page they will see:
They need to click on the aA button in the URL bar and select "Request Mobile Website":
They will then be presented with this page and will be redirected to the AppStore via the button.