The plugin doesn't seem to get properly loaded on iOS for debug or release builds. This can be reproduced with e.g. the "standard" example just by building it and trying to run it on an iOS device.

Running the example results in errors such as

qrc:/main.qml:38:1: static plugin for module "QtQuick.TreeView" with name "QQuickTreeViewPlugin" cannot be loaded: Cannot protect module QtQuick.TreeView 2 as it was never registered

QQmlApplicationEngine failed to load component
qrc:/main.qml:15:1: module "QtQuick.TreeView" is not installed