Iproperty setting Commit fails for fragmented mp4 files

I am trying to set PKEY_Media_Publisher for mp4 files.

This is how I am doing it (I don't have error checking here.):

IShellItem2 * shellItem = NULL;
IPropertyStore * propStore = NULL;
PROPVARIANT value;

hr = SHCreateItemFromParsingName(wchOutputVideoFile, NULL, IID_PPV_ARGS(&shellItem));

hr = shellItem->GetPropertyStore(GPS_READWRITE, __uuidof(IPropertyStore), (void**)&propStore);

value.vt = VT_LPWSTR;
value.pwszVal = L"My Publisher";

hr = propStore->SetValue(PKEY_Media_Publisher, value);

hr = propStore->Commit();

This works fine for regular mp4 files created with IMFSinkWriter.

However, for mp4 files created with MFCreateFMPEG4MediaSink the "Commit" call fails with E_FAIL.

Does anybody know why this might happen?