An investigation across ~50 accounts says Codex silently serves some gpt-6-astra requests from a weaker model.

From about 09:00 UTC on September 20, more than half the accounts tested got cruder output, around 25% less output per response, and faster token streams, while the server still reported gpt-6-astra (Fisher p = 2e-12). The effect follows the ChatGPT account, not the prompt or the client version (is-my-astra-real). The authors suspect gpt-5.6-luna as the substitute and say they can't prove it. A check script is included, and given the p-value I'd run it before assuming your own results are clean.