Why does NSURLSession with Multipath entitlement seamlessly switch to cellular when on a hardware Wi-Fi with no internet, but WKWebView does not?

Body:
Hi all, I’m seeing a puzzling discrepancy in behavior between NSURLSession (with multipathServiceType = NSURLSessionMultipathServiceTypeInteractive) and WKWebView when the device is connected to a Wi-Fi SSID that has no internet (e.g., a hardware device’s AP). I have the Multipath entitlement properly enabled, and in this scenario:

• NSURLSession requests automatically fall back to cellular and succeed (no user intervention, fast switch).
• WKWebView loads fail or stall: the web content does not appear, and it seems like the web view is not using the cellular path even though the system network path becomes satisfied and real Internet reachability is confirmed.

Environment:

• iOS version: (e.g., iOS 18.4)
• Device: (e.g., iPhone 15 Pro)
• Multipath entitlement: enabled in the app, using NSURLSessionMultipathServiceTypeInteractive
• Connected SSID: hardware device Wi-Fi with no external internet
• Expected fallback: automatic to cellular once the Wi-Fi has no internet, as observed with NSURLSession

What I’ve done / observed:

1. NSURLSession using Multipath works as expected:
NSURLSessionConfiguration *cfg = [NSURLSessionConfiguration defaultSessionConfiguration];
cfg.multipathServiceType = NSURLSessionMultipathServiceTypeInteractive;
NSURLSession *session = [NSURLSession sessionWithConfiguration:cfg];
NSURLRequest *req = [NSURLRequest requestWithURL:[NSURL URLWithString:@"https://www.apple.com/library/test/success.html"]];
NSURLSessionDataTask *task = [session dataTaskWithRequest:req completionHandler:^(NSData *data, NSURLResponse *resp, NSError *err) {
NSLog(@"NSURLSession result: %@, error: %@", resp, err);
}];
[task resume];
When connected to the device Wi-Fi (no external internet), the session quietly shifts to cellular and completes successfully.
2. WKWebView fails to load under the same conditions:
[self.webView loadRequest:[NSURLRequest requestWithURL:[NSURL URLWithString:@"https://www.apple.com/library/test/success.html"]]];
The web view either shows a load failure or just hangs, even though lower-level monitoring reports that the network path is satisfied and real Internet connectivity is available.
3. Network path monitoring logic:
   • I use the C API nw_path_monitor to watch for nw_path_status_satisfied.
   • Once satisfied is observed, I perform a true connectivity check using nw_connection (e.g., connecting tohttps://www.apple.com/library/test/success.html) to verify that real Internet traffic can flow over cellular.
   • That check passes, confirming fallback to cellular, but WKWebView still does not load content. Meanwhile, NSURLSession requests in the same condition succeed immediately.
4. Sample logging trace:
[+] nw_path_status_satisfied=1, hasWiFi=1, hasCellular=1
[+] Internet connectivity test: ready (via nw_connection)
[-] WKWebView load failed / stalled
[+] NSURLSession request completed successfully

Questions:

1. Why does NSURLSession with the multipath service type seamlessly use cellular when the Wi-Fi has no internet, but WKWebView does not exhibit the same fallback behavior? Is WKWebView not honoring the system’s multipath fallback the same way? Does it use a different networking stack or ignore the multipath entitlement in this scenario?
2. Is there a supported way to force WKWebView to behave like NSURLSession here?
   • For example, can I bridge content through a multipath-enabled NSURLSession and inject it into WKWebView via a custom scheme?
   • Are there any WKWebView configuration flags, preferences, or policies that enable the same automatic interface switching?
3. Are there known limitations or documented differences in how WKWebView handles network interface switching, path satisfaction, or multipath compared to raw NSURLSession?

What I’ve ruled out / tried:

• Verified the Multipath entitlement is included and active.
• Confirmed network path is “satisfied” and that real Internet reachability succeeds before calling [webView loadRequest:].
• Delayed the WKWebView load until after connectivity verification.
• Observed that NSURLSession requests succeed under identical connectivity conditions.

Any insight into internal differences, recommended workarounds, or Apple-recommended patterns for making web content robust in a “Wi-Fi with no internet” + automatic fallback-to-cellular scenario would be greatly appreciated. Thank you!