-[WKWebViewConfiguration copyWithZone] doesn't copy the _groupIdentifier string, which means WKWebView's created with a copy of the configuration won't have a page group. For example, this is done during UIDelegate::UIClient::createNewPage().
Created attachment 328633 [details]
Comment on attachment 328633 [details]
Fixed in <https://trac.webkit.org/changeset/225603/webkit>.