That looks like an aggressive chroma subsampling/quantization. Image encoders must allocate the bitrate among the channels, luma (Y), and chroma (Cb, Cr). Most

That looks like an aggressive chroma subsampling/quantization. Image encoders must allocate the bitrate among the channels, luma (Y), and chroma (Cb, Cr). Most of the time it is better to allocate most of the bitrate to the luma, because that's what the eye is most sensitive to. However it can create artifacts like this.