From Xcode 12 you should come across a very big problem that causes a compiling error and even a wrong behavior during the reading of the Qt project.
Waiting for an official solution by Qt, this is a quick guide to fix the issue.

Go to ios/mkspecs/features/toolchain.prf in the qt kit folder you are using.

In the lines 185 - 189 you will find cxx_flags for darwing os; add the argument -arch $$QMAKE_APPLE_DEVICE_ARCHS.

The result will be like this:

darwin {
    uikit:macx-xcode: \
        cxx_flags += -isysroot $$sdk_path_device.value -arch $$QMAKE_APPLE_DEVICE_ARCHS
    else: \
        cxx_flags += -isysroot $$QMAKE_MAC_SDK_PATH -arch $$QMAKE_APPLE_DEVICE_ARCHS

Reload the project in Qt Creator.