Permanent MFX_WRN_DEVICE_BUSY from EncodeFrameAsync

I've run across a condition where EncodeFrameAsync starts returning MFX_WRN_DEVICE_BUSY and it never gets unbusy. In my app (based on the multi transcode sample) this ends up creating an endless loop in EncodeOneFrame(...).

I can reproduce consistently when transcoding "29.97" soft telecined content while changing the framerate to 59.97. It works fine when converting interlaced 29.97 to 59.97 progressive.

Is this a driver issue or something that I'm missing?

Thanks