I'm able to circumvent around the issue by using this keyword which I copy-pasted from the test cases of the SSHLibrary, but I still think it would be nice to fix the actual issue:

Port Should Be Free
    [Arguments]  ${port}
    ${result}  Run Keyword If  os.sep == '/'
    ...  Run  netstat -tulpn
    ...  ELSE
    ...  Run  netstat -an
    Should Not Contain  ${result}  :${port}
    [Return]  ${result}

and then call Wait Until Keyword Succeeds 2 min 10s Port Should Be Free ${local_port} before calling the "Create Local Ssh Tunnel"

tiiuae/ci-test-automation@a790633

From the documentatin of socketserver:
"Finally, call server_close() to close the socket (unless you used a with statement).". So I think the line self.server.server_close() should be added after the shutdown to clean things up