Yes, CMS should do lossless optimizations no question. The problem here is that this is a photographic image, saved as a PNG, when it really should be a JPEG. Converting to JPEG is a lossy optimization, in addition to offering different features. It's risky to have a general "convert PNGs to JPEG" or even "convert all non-transparent PNGs to JPEG" optimization rules built into your build process or as a CMS plugin