I used recently installed Qt via open source online installer on MacOS.
I choose two versions of Qt in online installer: "Qt 5.12.7 for iOS" and "Qt 5.14.1 for iOS".

If I run code bellow in "Qt Cretor 4.11.1" with kit "Qt 5.12.7 for iOS" all works
fine:

import QtQuick.Window 2.12
import QtQuick 2.12
import QtQuick.Controls 2.12

Window {
    id: window
    visible: true
    width: 640
    height: 480
    title: qsTr("Hello World")

    Text {
        text: "Look at me"
    }

   Button {
        text: "Click me"
        onClicked: console.log("impressed")
    }
}

but if I run "Clean All" and choose another kit: "Qt 5.14.1 for iOS",
then I got:

QQmlApplicationEngine failed to load component
qrc:/main.qml:16:4: Button is not a type

and application closed right after start on iPad.

If I don't run "Clean All" application some time starts,
looks like "Qt Creator" for some reason run "Qt 5.12" version,
what is really strange because of "Qt Creator" puts them into different directories:

build-demo-Qt_5_12_7_for_iOS-Debug
build-demo-Qt_5_14_1_for_iOS-Debug

have no idea how it can mix up them.