Answering my own question. The issue: Affects both WKWebView and Safari Is not specific to iOS 18 Does not affect just local URLs; I can reproduce it during a port forwarding session to an external domain How to reproduce with Nextjs which is the framework I'm using: Install a self-signed certification for a domain like Start a Nextjs dev server Open any page in the Nextjs dev server in Safari on a Simulator Open Safari Dev Tols The Elements tab will be either empty or show only the very top level elements of the DOM. The Network and Sources tab will show a correct list of resources that have been loaded but won't show the responses as you'd normally see.

Also if the simulator is on iOS 18 and the WKWebView loads an external URL, it's fine. As far as I can tell, the issues are only with a local URL

For anyone else who comes across this in the future... Once you upgrade some of your WKWebViews by using WKAppBoundDomains, other domains are downgraded and lose features. A good example is evaluateJavaScript. It works in any WKWebView by default. But as soon as you create a WKAppBoundDomains array in your Info.plist, the default changes and becomes more strict. Now, using evaluateJavaScript has two requirements: the domain of the WKWebView must be in the app-bound domains array and WKWebViewConfiguration.limitsNavigationsToAppBoundDomains must be true. If a WKWebView tries to use evaluateJavaScript but does not meet both of those requirements, it will fail loading.