H.264 adding dark anti-aliasing even on high444 yuv444p
It's a rendering artifact, not an actual error.

See

ffplay test444.mp4 -vf scale=iw*16:-1:flags=neighbor


and

ffmpeg -i test444.mp4 roundtrip.png


You should see no black pixels.

Update: ffplay downsamples YUV inputs to 420 before final conversion to RGB.

[swscaler @ 0000000005a82800] bicubic scaler, from yuv444p to yuv420p using MMXEXT


You can avoid artifacts by directly calling pixel format filter.

ffplay test444.mp4 -vf format=bgra