Scenario:

QT is used in c:\foo\a.dll that uses Qt Multimedia. c:\bar\test.exe loads the dll. A sample repro skeleton is available at https://github.com/lwahonen/qt-dll

After running windeployqt c:\foo\a.dll, Qt6Multimedia.dll is deployed to c:\foo and the backend is placed in c:\foo\multimedia\ffmpegmediaplugin.dll

Unfortunately loading ffmpegmediaplugin.dll fails because it's dependent on Qt6Multimedia.dll which is not found in neither the executable directory ( c:\bar ) nor in the same directory as backend itself ( c:\foo\multimedia )

This can be worked around by adding c:\foo to dll search path with SetDllDirectory:

QString currentDir = QString::fromStdWString(getCurrentDllFolder()); <- returns c:\foo
app.addLibraryPath(currentDir);
SetDllDirectory(getCurrentDllFolder().c_str());