Setting QTC_DEBUGGER_MULTIPROCESS=1 and launching Qt Creator shows "Enable Debugging of Subprocesses" under Debugger Settings of the Run configuration.

If starting a debug session for a remote Generic Linux device, this checkbox always seems to get re-checked again. As a result, the GDB detach-on-fork option is always set to off.

This has the effect that starting a process using QProcess can hang or timeout.

For example this can hang:
QProcess::execute("/bin/echo", QStringList() << "hello world")