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Description
Appcenter's install page is attempting to determine if a mobile browser is in use or not in order to make available the download and installation functionality.
If a user has turned on "request desktop website" in Safari as an option, this blocks any downloading and installing on the appcenter website.
iOS 13.3 has turned this feature on by default on iPads.
Thus anyone who upgrades to iOS 13.3 will suddenly find they can no longer install off of the appcenter website and there is no clear path for them to figure it out.
You will need to adjust to this new fact of life by a better check for mobile Safari than what you're using (good luck since Apple seems to be intentionally obfuscating it).