We have now resolved this, it was an issue with the app prefix id.
JW-SDG
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We have resolved this. we are still using the old format which according to the documentation allows us to specify the app identifier using either the TeamID or the App Prefix ID. we have always used the teamID.
The new format only allows the use of the App Prefix ID, but it turns out that after some update recently (i suspect it might have been 17.3 based on some data we have) even the old format won't work unless the App Prefix ID is used. we changed the App-association file to use the App Prefix ID instead and it started working.
To further Clarify this. If you tap back while the page you are on is still loading (the html can be fully loaded but additional resources are still coming down) the you get this error. this is a "Cancelled" error, which is ok, and the web view reports that it is still loading the page you are returning to.
However, if the page you are returning to wasn't fully loaded when you moved on, then the error is thrown again and the web view stops loading.
in this situation, within
func webView(_ webView: SDWebView, didFail navigation: WKNavigation!, withError error: Error)
if the webview is not loading and a .reload() is called, the page will load.
This behaviour has only started happening since iOS 13.4