According to configuration manual: http://cbonte.github.io/haproxy-dconv/2.0/configuration.html#4.2-option%20http-keep-alive
If the client request has to go to another backend or another server due to
content switching or the load balancing algorithm, the idle connection will
immediately be closed and a new one re-opened.
But connections to backend servers are not closed after another server is chosen for sending request. As mentioned in observation: HAProxy http-reuse never option not working for HAProxy 1.9.8 and 2.0.1