I am hitting the same issue.
Background
Jenkins is running in an on premise kubernetes cluster.
NAME READY STATUS RESTARTS AGE IP NODE
jenkins-0 1/1 Running 0 5s 10.43.0.2 nem-docker-app-node05.inter-olymp.local
There are two service exposed as NodePort. One for jenkins using NodePort 30100 and one for the slave port using 30200, which is set to 50000
NAME TYPE CLUSTER-IP EXTERNAL-IP PORT(S) AGE SELECTOR
jenkins NodePort 10.96.191.11 <none> 8080:30100/TCP 8s app=jenkins
jenkins-agents NodePort 10.108.78.226 <none> 50000:30200/TCP 8s app=jenkins
I browse to my Jenkins instance using port 30100, e.g. http://build.mydomain.com:30100 and create a new permanent agent called test.
What I Expected
Execute the following command and the slave agent connects to Jenkins.
java -jar agent.jar -jnlpUrl http://build.mydomain.com:30100/computer/test/slave-agent.jnlp -secret c80dc7812095e908d9807b83dbb7406f03f8274f83f70ca407e9fcec2665ed7a -workDir "C:\Jenkins"
What Happened
I get a 404 not found.
Oct 19, 2018 9:57:45 AM org.jenkinsci.remoting.engine.WorkDirManager initializeWorkDir
INFO: Using C:\Jenkins\remoting as a remoting work directory
Both error and output logs will be printed to C:\Jenkins\remoting
Failing to obtain http://build.mydomain.com:30100/computer/test/slave-agent.jnlp?encrypt=true
java.io.IOException: Failed to load http://build.mydomain.com:30100/computer/test/slave-agent.jnlp?encrypt=true: 404 Not Found
at hudson.remoting.Launcher.parseJnlpArguments(Launcher.java:496)
at hudson.remoting.Launcher.run(Launcher.java:322)
at hudson.remoting.Launcher.main(Launcher.java:283)
Additional Info
Browsing to http://build.mydomain.com:30200>/ gives
Jenkins-Agent-Protocols: JNLP4-connect, Ping
Jenkins-Version: 2.121.3
Jenkins-Session: bb58e25a
Client: 10.42.0.0
Server: 10.43.0.2
Remoting-Minimum-Version: 2.60
So I am not sure what is going on here. The agent port is reachable through NodePort 30200. It sends traffic to 50000. But the slave-agent.jnlp is not found 
Same thing here. Is there a workaround for this?