The iio-sensor-proxy backend appears to have a blocking DBus call on initialization, which can cause Qt to freeze potentially due to underlying backend bugs. 

https://github.com/qt/qtsensors/blob/ff903c10cfdd0651ad5c1ae80a4fd7456badd759/src/plugins/sensors/iio-sensor-proxy/iiosensorproxyproximitysensor.cpp#L33

This can be reproduced by clicking the button on this simple example, with a OnePlus 6 and postmarketOS:

import QtQuick
import QtQuick.Controls
import QtSensors

Item {    

    Component.onCompleted: console.log('hello')    

    ProximitySensor {
       id: sensor
       active: false
       onReadingChanged: console.log(reading)
   }

   Button {
       anchors.centerIn: parent
       text: 'hello'
       onClicked: sensor.start()
   }

{color:#000000}}

The entire app freezes after `sensor.start()` is called.