The case is solved. Anthropic urgently revoked Fable5's access, most likely due to a betrayal by Amazon, which tipped off the U.S. government.

Amazon researchers kept challenging Fable 5, conducting extreme jailbreak tests. They used a series of special prompt words to continuously induce the model to answer questions it shouldn't have, ultimately causing the model to leak some security vulnerability-related information.
Subsequently, they compiled their research findings into a security report, and the U.S. government then imposed restrictions on Fable 5 and Mythos 5 citing national security concerns.