Think it's an old bug in the SDK, but figured I'd post it because it persists with the redesign.
User thumbnails for worlds + avatars display differently in the SDK than everywhere else.
I could approximate the change by applying [Unity's] sRGB > Linear LUT to one of my thumbnails in external software - so I'm leaning toward it being an issue with color space! Unity could be taking the fetched sRGB texture/image + doing a transform to Linear (or is just treating it as Linear).
I'm not a programmer at all, but this seemed hyper-relevant.