To make sure I wasn't the cause of that issue, I tried again on a fresh QML project created from QtCreator (full code at the bottom).
I am having the problem for single and multi-threaded WASM builds.

I have found this corrected bug possibly related to my issue, however:

• I am on Qt 6.11.1 (emsdk 4.0.7); I have also tried on Qt6.7.3 (emsdk 3.1.50) with the same result.
  Both versions are supposed to be free of that bug.
• The working snippet provided by the reporter (setting nameFilter to "*") does not work.

Is anyone reproducing this issue / able to get the dialog with the code below?
Or would you know what I am doing wrong perhaps?

#include <QtWidgets/QApplication>
#include <QQmlApplicationEngine>

#include <QtWidgets/QFileDialog>

int main(int argc, char *argv[])
{
    QApplication app(argc, argv);

    auto fileContentReady = [](const QString &fileName, const QByteArray &fileContent) {
        if (fileName.isEmpty()) {
            // No file was selected
            qDebug() << fileName;
        } else {
            // Use fileName and fileContent
            qDebug() << fileName;
        }
        QFileInfo fi(fileName);
        qDebug() << "path: " << fi.path();

    };
    QFileDialog::getOpenFileContent("*",  fileContentReady);

    QQmlApplicationEngine engine;
    QObject::connect(
        &engine,
        &QQmlApplicationEngine::objectCreationFailed,
        &app,
        []() { QCoreApplication::exit(-1); },
        Qt::QueuedConnection);
    engine.loadFromModule("wasm", "Main");

    return QCoreApplication::exec();
}