Maybe a problem with your ffmpeg binary. What does info() report ?
Code:
Blankclip(pixel_type="YV12").KillAudio()
convertbits(16)
ConverttoYUV444()
Prefetch(4)
ConvertBits(bits=10, dither=1)
For this test script, info() reports YUV444P10 ; and ffmpeg -i input.avs reports yuv444p10le
Why don't you have ConvertBits before prefetch ?