This is a brand new exception discovery when SoapClient uses cached wsdl-data and there are for example a HTTP 401 Unauthorized response from the API.
As SoapClient has already fetched the WSDL document, SoapClient are no longer authorizing together when the wsdl is downloading and store locally. The authorization is delayed until the real wsdl request is executed.
netcurl and the used SoapClient saved a lot of time here (up to 2-3 seconds) but this also moved the authfail to another point of request.
This won't (and can't) be fixed in 6.0, but for 6.1 this is solved.