I'm not given the popup on iOS to open my app via URI scheme anymore, why?

Apple enforces a popup message (disambiguation dialog) when opening an app via URI scheme. If you had previously seen this popup while testing or otherwise, but are no longer seeing the option to click on "Open", it's possible that you had clicked on "Cancel" and that response has been cached by Safari, so you won't be prompted again.

In this case, clear your Safari cache, click the link again, and the popup should reappear, allowing you to open the app.