The HttpClientOptions introduced in 5.8.0 caused a misconfiguration in the underlying HTTP Client by trying to be too fancy (I'm sorry). (#466)
This is technically a breaking change because the return type of some public methods of the HttpClientOptions has changed - but since they are meant to be used for service creation, it is very unlikely that they have been used outside the SDK internals, so the risk of breaking an existing application with this change is so low that I'll take the risk of getting shouted at for it.