I've discovered that if my QtCreator project has a path that is over a certain length, that the build never completes. It's happening for the 'Boot2Qt 5.15.5 Raspberry Pi 3' kit. Windows Desktop minGW kit builds compile OK. The 'Compile Output' continually outputs the following line.
Examining Windows Task Manager shows QtCreator has spawned multiple (can be hundreds) instances of make.exe processes from the raspberrypi3 pokysdk-mingw32 toolchain.
I've tried enabling the HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Control\FileSystem key LongPathsEnabled but that makes no difference.
If I move my Creator project to a location with a shorter file path then the build completes. Interestingly in this case the path used my qmake to the .pro file is absolute, not relative in the case where the build fails to complete.
I've included copies of my projects. The project sub-folder in MyFolderOne compiles, but the project in sub-folder MyFolderTwo does not.