Client Hints are a set of HTTP request header fields that a server can proactively request from a client to get information about the device, network, user, and user-agent-specific preferences. Ideally, image optimization tooling should be willing to modify their responses based on the result of these hints (i.e. serve smaller images to those on slower networks, etc).
Did the service include an accept-ch header in their response? If so, did it respond with differing content based on different inputs to the requested headers?