Você está perdendo a opção redispatch
, aparentemente.
Na documentação :
option redispatch / no option redispatch: Enable or disable session redistribution in case of connection failure
In HTTP mode, if a server designated by a cookie is down, clients may definitely stick to it because they cannot flush the cookie, so they will not be able to access the service anymore.
Specifying "option redispatch" will allow the proxy to break their persistence and redistribute them to a working server.