spring-cloud-gateway: Empty CloseWebSocketFragment leads to sending an invalid CloseStatus 1005

Recently we are seeing the following exception sporadically in our logs. It seems like org.springframework.web.reactive.socket.adapter.ReactorNettyWebSocketSession is not working correctly if the io.netty.handler.codec.http.websocketx.CloseWebSocketFrame is empty:

Stack Trace:
  reactor.core.Exceptions$ErrorCallbackNotImplemented: java.lang.IllegalArgumentException: WebSocket close status code does NOT comply with RFC-6455: 1005
Caused by: java.lang.IllegalArgumentException: WebSocket close status code does NOT comply with RFC-6455: 1005
        at io.netty.handler.codec.http.websocketx.CloseWebSocketFrame.requireValidStatusCode(CloseWebSocketFrame.java:209)
        at io.netty.handler.codec.http.websocketx.CloseWebSocketFrame.<init>(CloseWebSocketFrame.java:69)
        at reactor.netty.http.client.WebsocketClientOperations.sendClose(WebsocketClientOperations.java:213)
        at org.springframework.web.reactive.socket.adapter.ReactorNettyWebSocketSession.close(ReactorNettyWebSocketSession.java:124)
        at reactor.core.publisher.MonoFlatMap$FlatMapMain.onNext(MonoFlatMap.java:125)
        at reactor.core.publisher.FluxFilter$FilterSubscriber.onNext(FluxFilter.java:113)
        at reactor.core.publisher.FluxMap$MapConditionalSubscriber.onNext(FluxMap.java:220)
        at reactor.core.publisher.FluxFirstWithSignal$FirstEmittingSubscriber.onNext(FluxFirstWithSignal.java:330)
        at org.springframework.cloud.sleuth.instrument.reactor.ScopePassingSpanSubscriber.onNext(ScopePassingSpanSubscriber.java:88)
        at reactor.core.publisher.Operators$MonoInnerProducerBase.complete(Operators.java:2634)
        at reactor.core.publisher.SinkOneMulticast.tryEmitValue(SinkOneMulticast.java:70)
        at reactor.netty.http.server.WebsocketServerOperations.sendCloseNow(WebsocketServerOperations.java:258)
        at reactor.netty.http.server.WebsocketServerOperations.onInboundNext(WebsocketServerOperations.java:156)
        at reactor.netty.channel.ChannelOperationsHandler.channelRead(ChannelOperationsHandler.java:93)
        at io.netty.channel.AbstractChannelHandlerContext.invokeChannelRead(AbstractChannelHandlerContext.java:379)
[...]

I already discussed this in detail in a bug report for Reactor Netty: https://github.com/reactor/reactor-netty/issues/1891 @violetagg was able to convince me that Reactor Netty is doing everything right at this point and that the problem lies in the way org.springframework.cloud.gateway.filter.WebsocketRoutingFilter uses ReactorNettyWebSocketSession