Encountered this problem when attempting echo mutli-parts message. The server side is a simple echo endpoint in glassfish.
Scenarios:
Websocket.sendText("first", false).get();
Websocket.sendText("last", true).get();
// Then wait for onText in customized listener.
I attached a webrev for it

It is in sandbox http-client-branch only, no such problem with jdk 11 promotion builds.