A previous patch (https://github.com/apache/thrift/pull/2002) for https://issues.apache.org/jira/browse/THRIFT-5084 added multiplexing support but contained a logical error when checking the message type. It should allow `oneway` and `call` message types and reject all others, but it rejected `oneway` and `call` messages and accepted all others.
I've fixed the bug and updated the unit tests. GitHub PR linked. My apologies for any inconvenience caused.