The following two API tests are consistently asserting on iOS simulator debug bots after https://commits.webkit.org/256048@main landed:

TestWebKitAPI.CopyHTML.SanitizationPreservesCharacterSet
TestWebKitAPI.CopyHTML.SanitizationPreservesCharacterSetInSelectedText

        ASSERTION FAILED: m_clients.contains(&client)
        platform/graphics/FontCache.cpp(400) : void WebCore::FontCache::removeClient(WebCore::FontSelector &)
        1   0x11c7423d9 WTFCrash
        2   0x173fc22eb WTFCrashWithInfo(int, char const*, char const*, int)
        3   0x178ec7f36 WebCore::FontCache::removeClient(WebCore::FontSelector&)
        4   0x177529c61 WebCore::CSSFontSelector::~CSSFontSelector()
        5   0x177529f25 WebCore::CSSFontSelector::~CSSFontSelector()
        6   0x177529f89 WebCore::CSSFontSelector::~CSSFontSelector()
        7   0x17707d4bc std::__1::default_delete<WebCore::FontSelector>::operator()(WebCore::FontSelector*) const
        8   0x17707d487 WTF::RefCounted<WebCore::FontSelector, std::__1::default_delete<WebCore::FontSelector> >::deref() const
        9   0x17752dd59 WebCore::CSSFontSelector::deref()
        10  0x177534c49 WTF::Ref<WebCore::CSSFontSelector, WTF::RawPtrTraits<WebCore::CSSFontSelector> >::~Ref()
        11  0x177529325 WTF::Ref<WebCore::CSSFontSelector, WTF::RawPtrTraits<WebCore::CSSFontSelector> >::~Ref()
        12  0x177a1bdc9 WebCore::Document::~Document()
        13  0x177f8342f WebCore::HTMLDocument::~HTMLDocument()
        14  0x177f83455 WebCore::HTMLDocument::~HTMLDocument()
        15  0x177f83539 WebCore::HTMLDocument::~HTMLDocument()
        16  0x177a1e673 WebCore::Document::decrementReferencingNodeCount()
        17  0x177a1df47 WebCore::Document::removedLastRef()
        18  0x177c0b034 WebCore::Node::removedLastRef()
        19  0x17412c8dc WebCore::Node::deref() const
        20  0x174fe3a7e WTF::DefaultRefDerefTraits<WebCore::Document>::derefIfNotNull(WebCore::Document*)
        21  0x174fe3a49 WTF::RefPtr<WebCore::Document, WTF::RawPtrTraits<WebCore::Document>, WTF::DefaultRefDerefTraits<WebCore::Document> >::~RefPtr()
        22  0x174fe37c5 WTF::RefPtr<WebCore::Document, WTF::RawPtrTraits<WebCore::Document>, WTF::DefaultRefDerefTraits<WebCore::Document> >::~RefPtr()
        23  0x178726157 WebCore::DocumentWriter::begin(WTF::URL const&, bool, WebCore::Document*, WebCore::ProcessQualified<WTF::UUID>, WebCore::NavigationAction const*)
        24  0x17871d96d WebCore::DocumentLoader::commitData(WebCore::SharedBuffer const&)
        25  0x1262426d3 -[WebFrame(WebInternal) _commitData:]
        26  0x12631ddc6 -[WebHTMLRepresentation receivedData:withDataSource:]
        27  0x12622c15a -[WebDataSource(WebInternal) _receivedData:]
        28  0x12622301c WebFrameLoaderClient::committedLoad(WebCore::DocumentLoader*, WebCore::SharedBuffer const&)
        29  0x17872526e WebCore::DocumentLoader::commitLoad(WebCore::SharedBuffer const&)
        30  0x178726e13 WebCore::DocumentLoader::dataReceived(WebCore::SharedBuffer const&)
        31  0x17875ff90 auto WebCore::DocumentLoader::continueAfterContentPolicy(WebCore::PolicyAction)::$_9::operator()<WTF::Ref<WebCore::SharedBuffer, WTF::RawPtrTraits<WebCore::SharedBuffer> > >(WTF::Ref<WebCore::SharedBuffer, WTF::RawPtrTraits<WebCore::SharedBuffer> >&&) const
        Child process terminated with signal 11: Segmentation fault

https://results.webkit.org/?suite=api-tests&suite=api-tests&test=TestWebKitAPI.CopyHTML.SanitizationPreservesCharacterSetInSelectedText&test=TestWebKitAPI.CopyHTML.SanitizationPreservesCharacterSet