Netty4TcpChannel Negative longs unsupported - repeated in logs

Elastic Stack Elasticsearch
6

and yesterday:

zcat elk-2020-02-12-1.log.gz | grep 2020-02-12 | grep -c "send mess"
3957
zcat elk-2020-02-12-1.log.gz | grep 2020-02-12 | grep -c "failure parsing pipeline config"
1234
zcat elk-2020-02-12-1.log.gz | grep 2020-02-12 | grep -c "failed to execute on node"
45
zcat elk-2020-02-12-1.log.gz | grep 2020-02-12 | grep -c "failed to execute pipeline"
2908
zcat elk-2020-02-12-1.log.gz | grep 2020-02-12 | grep "send mess" | head
[2020-02-12T20:23:21,159][WARN ][o.e.t.OutboundHandler    ] [ela1] send message failed [channel: Netty4TcpChannel{localAddress=/10.20.132.93:9300, remoteAddress=/10.20.132.94:42874}]
[2020-02-12T20:24:55,403][WARN ][o.e.t.OutboundHandler    ] [ela1] send message failed [channel: Netty4TcpChannel{localAddress=/10.20.132.93:9300, remoteAddress=/10.20.132.94:43194}]
[2020-02-12T20:24:58,189][WARN ][o.e.t.OutboundHandler    ] [ela1] send message failed [channel: Netty4TcpChannel{localAddress=/10.20.132.93:9300, remoteAddress=/10.20.132.94:43190}]
[2020-02-12T20:25:05,159][WARN ][o.e.t.OutboundHandler    ] [ela1] send message failed [channel: Netty4TcpChannel{localAddress=/10.20.132.93:9300, remoteAddress=/10.20.132.94:43192}]

right after upgrade... of course it can be something that starts with elastic restart too