This probably doesn’t have anything to do with VoltBuilder - we just call the Cordova libraries, which in turn call XCode.
More likely, it has to do with the change from UIWebView to WKWebView as required by Apple. (PhoneGap used UIWebView, but never updated). You might want to research how localStorage works in WKWebView.
I found this to get you started:
Here’s a item of possible interest in the cordova-ios repo: