Are you using Chromecast to stream to the Onkyo? If so, I think it automatically downsamples over Chromecast unless the receiving device says it can receive hi-res. Many audio systems with “Chromecast built-in” don’t seem to do that properly, and I’ll bet the Onkyo is one of them. Onkyo is all about the DTS Play-Fi; I don’t think they take the Chromecast stuff seriously. Might check for an Onkyo firmware update?
You coulld also get a streamer, like the Allo DigiOne, and stream into the Onkyo via its S/PDIF connectors. That should give you full resolution.
Turns out one is for sale: