I have a problem with websocket (PlayFramework) under stress test

Hello.I using Playframework 2.6.I created a microservice, which uses websockets. I decided to do the stress test for result 101 (Switching Protocols).When I doing 20-30 RPS , then Ok. But ,if I doing more then 30,then I gote 46% errors - Unmasked client frame(error of protocol).I want to note that the web service developed on Spring Reactive works without problems under the given conditions. Why is this happening?