The BE-75 can only output 10bit, but internally uses 12bit processing to maintain signal accuracy.
Are you using AVIsynth+ ver3.6.? Sounds like you need to update QTGMC as well if you are still using YUY2?
You need the pinterf versions of all the QTGMC dependencies. Yes, I too used M.Express.

I don't know what AVIsynth+ version I'm using, I remember it was a nightmare to get the whole thing to work error free, I sure hope I don't have to go through that again, I didn't mind converting to YUV2 because I have to crop to 704 using vdub2 so in the process I save as YUV2 but it would be nice if I can get QTGMC and FFMPEG to work without YUV2.