Cannot debug QML when attaching to existing process

Both Build and Run configurations of my project are set up for debugging C++ and QML.

If I click the Debug button on the left (Start debugging of startup project) I can debug both C++ and QML.

However if I click Run, and then once the app is running I click the Debug button on the Application Output panel (Attach debugger to PID 12345) then I get a popup warning that certain breakpoints won't be used and that for QML debugging I need to set this in the Build and Run configurations. And sure enough QML debugging is not available.