The configuration seems ok.
According to single get() failover doesnt work! · Issue #154 · php-memcached-dev/php-memcached · GitHub it seems the behaviour is kind of expected because you might be hitting one of the down servers and an error could happen.
If this is true, I guess there is no solution on our side at the moment, and I’m not sure if ownCloud can provide a solution for it because of the same reasons the php-memcached guys argue.
I don’t know if redis has the same problem, but you might give it a try.