Bad artifacts on very low complex pictures when doing xavc encoding

I tried to use xavc branch (https://code.videolan.org/ifb/x264) to encoding xavc class 300 4K video, but found some bad artifacts on very low complex pictures.

The source YUV file: https://drive.google.com/file/d/1VgiigAlUmnp6YO3NyRpWfs3g39sb41Td/view?usp=sharing

Command line: x264cli -o x264_test_sample.264 --input-fmt raw --input-res 3840x2160 --fps 50 --input-depth 10 --input-csp i422 --output-depth 10 --output-csp i422 --preset ultrafast --tune fastdecode --avcintra-flavor sony --avcintra-class 300 x264_test_sample.yuv

This is not just occurred with xavc encoding, intra encoding with high bitrate also have same artifacts.