Update! I’ve found where in our settings we were mashing the images, and we are now NOT mashing them. 
Again noting that I chose this image because of the super-noisy clouds and gradients galore, and NOT because I think this is an especially pretty picture, but I love that it’s not being chewed up.
For the record, there is pixel-accurate lossless compression on the largest images. Our admin tools include a wipe tool that allows you to slide it back and forth to compare, and I can say that on a full-size, very bright 4K display, I really can’t see a difference. Click on it to see full size in the lightbox.
And of course see what you see with your own images!