I have recently upgraded my app to WKWebView and immediately ran into this issue.
My app has a simple download functionality that pulls PDFs from our server and saves the PDFs to File.dataDirectory. Before upgrading to WKWebView, this worked 100 percent of the time. After using WKWebView, this download step works perfectly fine in the simulator and immediately after downloading the app from the app store.
However, when I try to run the download step immediately after a Pro Deploy, the File.writeFile method hangs without any warning or error. If I kill the app, restart it, and try again, everything works perfectly fine.
Does anyone have an idea as to why it would fail only after Pro Deploy. Something with data directory initialization?
My related bug report can be found here: https://github.com/ionic-team/ionic-native/issues/2067