IMFPluginControl not working for MFVideoFormat_H264

I have a custom MFT that is registered for MFVideoFormat_H264. But this MFT is not picked up during normal topology resolution. Instead the topology loader always picks "Microsoft H264 Video Decoder MFT" ({62CE7E72-4C71-4D20-B15D-452831A87D9D}).

I'm using IMFPluginControl like this:

pluginControl->SetPreferredClsid(MF_Plugin_Type_MFT, L"{34363248-0000-0010-8000-00AA00389B71}", &myId);
pluginControl->SetDisabled(MF_Plugin_Type_MFT, __uuidof(CMSH264DecoderMFT), TRUE));

Setting just the preferred CLSID should be enough (and works for other formats). But even explicitly disabling the MS decoder has no effect.

Any ideas what's wrong?