Shop Pay WebView Error "Eligible: False" React Native

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I have Shop Pay enabled on our Shopify store. 

It works perfectly everywhere, other than in our iOS mobile app. 
THE PROBLEM: After users enter the 6 digit text verification message for Shop Pay they are shown the following page/error (see screenshot below): {"eligible":false}  
The iOS app is a react native app built with WebView (WKWebView) and has a custom user agent. 
Can I have support from your developer team to understand the reason for the error?