@Alexuds79 In case anyone finds the same problem in the future, using QtWebView (https://doc.qt.io/qt-5.15/qtwebview-index.html) instead of QtWebEngineView seems to solve the problem on MacOS.
However, on Windows (again with the final application built), it seems that QtWebEngineView works perfectly but QtWebView does not (the WebView QML component is not shown).
Fortunately, with simple logic you can use one or the other depending on the platform.