ffdshow isn't called if MPC is using Overlay Mixer?

In ffdshow, the Overlay page says that it supports Overlay Mixer and WMR9 in windowed mode, however it only seems to work if Media Player Classic is set to Overlay Mixer. In WMR9 Windowed, all of the sliders except Brightness are greyed out and even changing the brightness doesn't seem to affect the picture. They do work when MPC is set to Overlay Mixer, but here's the problem; Half the time, ffdshow is never called, even though I'm playing an Xvid file and I have Xvid set to libavcodec on the codecs page. Set MPC to WMR9 and ffdshow is called. Change the output to Overlay Mixer, re-open the file and ffdshow closes. It works with some files, but not most. Why does the renderer selected in MPC affect whether or not ffdshow is called to decode the file? How can I make sure that it's called every time regardless of the renderer?

That shouldn't be happening. I always use Overlay and I never had such a problem. Which decoder gets used instead of ffdshow when this happens? Did you make any changes to the output options in ffdshow?

Try disabling the whitelist & blacklist options. There might be something weird interfering (though it shouldn't be).

(in recent versions) ffdshow audio/video decoder configuration -> DirectShow Control