ffmpeg H264_qsv encoding generates horizontal green bar at the bottom of the video

hi there, 

I am using ffmpeg 4.1.3 H264_qsv encoder to capture a video from  camera at 1920 x 1080.

the camera outputs BGRA frame, and I use  IPP function ippiBGRToYCbCr420_8u_AC4P2R to convert it to NV12 format. then the NV12 frame is passed to H264_qsv  encoder. the video file extension is mp4. 

however, the video has a horizontal green bar at the bottom of video. the height of the bar is a few pixels and the width is across the full frame. 

does anyone have some advices how to debug the problem?

thank you and regards

Leon